Recombination mechanisms of luminescence type gas sensors.

Luminescence type gas sensors based on organic luminophores are characterized by an excellent signal stability over the luminophores' lifetime. Even though the sensing material is prone to degradation due to photobleaching, evaluation of the dynamic optical response allows to minimize aging induced drift effects of the luminophore and the optoelectronic components. The gas dependent luminophore decay time is mostly independent of the excitation intensity, which is attributed to the monomolecular recombination in many organic luminophores. To improve the overall sensor lifetime and to utilize this luminescence based sensor concept for long-term sensing applications, new luminophores are needed. Potential candidates are inorganic, semiconducting materials, which, however, show more complex recombination behaviour than the organic luminophores mentioned before, involving also bimolecular recombination. In the scope of this work, the differences of mono- and bimolecular recombination are discussed by the use of a simple statistical model. The theoretical aspects are furthermore confirmed by dynamic luminescence measurements on manganese doped zinc sulfide (ZnS). The semiconducting ZnS is an oxygen sensitive luminophore, which shows both, mono- and bimolecular recombination, depending on the excitation energy.

Multi reflection of Lamb wave emission in an acoustic waveguide sensor.

Recently, an acoustic waveguide sensor based on multiple mode conversion of surface acoustic waves at the solid-liquid interfaces has been introduced for the concentration measurement of binary and ternary mixtures, liquid level sensing, investigation of spatial inhomogenities or bubble detection. In this contribution the sound wave propagation within this acoustic waveguide sensor is visualized by Schlieren imaging for continuous and burst operation the first time. In the acoustic waveguide the antisymmetrical zero order Lamb wave mode is excited by a single phase transducer of 1 MHz on thin glass plates of 1 mm thickness. By contact to the investigated liquid Lamb waves propagating on the first plate emit pressure waves into the adjacent liquid, which excites Lamb waves on the second plate, what again causes pressure waves traveling inside the liquid back to the first plate and so on. The Schlieren images prove this multi reflection within the acoustic waveguide, which confirms former considerations and calculations based on the receiver signal. With this knowledge the sensor concepts with the acoustic waveguide sensor can be interpreted in a better manner.

Efficient semi-analytical simulation of elastic guided waves in cylinders subject to arbitrary non-symmetric loads.

In this paper, we present an approach to model the propagation of high-frequency elastic guided waves in solid or hollow cylinders. This formulation requires only discretization of the radial direction, whereas the circumferential direction is approximated via a truncated Fourier series, and the axial direction is described analytically. The model is extended to allow applying arbitrary non-symmetric loads f(r,θ) on the flat cylinder surface. Efficiency is increased by a proposed methodology to limit the number of Fourier coefficients and by the implementation of hierarchical shape functions to dynamically adjust discretization with respect to frequency. Results are validated against conventional finite element applications, demonstrating the accuracy of the model and a reduction of the computing time by three orders of magnitude. Additionally, we apply a matrix function solution for the scaled boundary finite element method leading to a linear solution in the static case.

Guided ultrasonic waves for determining effective orthotropic material parameters of continuous-fiber reinforced thermoplastic plates.

Ultrasonic methods are widely established in the NDE/NDT community, where they are mostly used for the detection of flaws and structural damage in various components. A different goal, despite the similar technological approach, is non-destructive material characterization, i.e. the determination of parameters like Young's modulus. Only few works on this topic have considered materials with high damping and strong anisotropy, such as continuous-fiber reinforced plastics, but due to the increasing demand in the industry, appropriate methods are needed. In this contribution, we demonstrate the application of laser-induced ultrasonic Lamb waves for the characterization of fiber-reinforced plastic plates, providing effective parameters for a homogeneous, orthotropic material model.

Quantum 1/f effect in resonant biochemical piezoelectric and MEMS sensors.

Piezoelectric sensors used for the detection of chemical agents and as electronic nose instruments are based on bulk and surface acoustic wave resonators. Adsorption of gas molecules on the surface of the polymer coating is detected by a reduction of the resonance frequency of the quartz disk, subject also to fundamental quantum 1/f frequency fluctuations. The quantum 1/f limit of detection is given by the quantum 1/f formula for quartz resonators. Therefore, for quantum 1/f optimization and for calculation and improvement of the fundamental sensitivity limits, we must avoid closeness of the crystal size to the phonon coherence length, which corresponds to the maximum error and minimal sensitivity situation, as shown here. Adsorbed masses below the pg range can be detected. Microelectromechanical system (MEMS) resonators have provided a possibility for the nanominiaturization of these sensors. Essential for integrated nanotechnology, these resonant silicon bars (fingers) are excited magnetically or electrically through external applied forces, since they are not piezoelectric or magnetostrictive. The application of the quantum 1/f theory to these systems is published here for the first time. It provides simple formulas that yield much lower quantum 1/f frequency fluctuations for magnetic excitation, in comparison with electrostatically driven MEMS resonators.

Reliable computation of roots in analytical waveguide modeling using an interval-Newton approach and algorithmic differentiation.

For the modeling and simulation of wave propagation in geometrically simple waveguides such as plates or rods, one may employ the analytical global matrix method. That is, a certain (global) matrix depending on the two parameters wavenumber and frequency is built. Subsequently, one must calculate all parameter pairs within the domain of interest where the global matrix becomes singular. For this purpose, one could compute all roots of the determinant of the global matrix when the two parameters vary in the given intervals. This requirement to calculate all roots is actually the method's most concerning restriction. Previous approaches are based on so-called mode-tracers, which use the physical phenomenon that solutions, i.e., roots of the determinant of the global matrix, appear in a certain pattern, the waveguide modes, to limit the root-finding algorithm's search space with respect to consecutive solutions. In some cases, these reductions of the search space yield only an incomplete set of solutions, because some roots may be missed as a result of uncertain predictions. Therefore, we propose replacement of the mode-tracer approach with a suitable version of an interval- Newton method. To apply this interval-based method, we extended the interval and derivative computation provided by a numerical computing environment such that corresponding information is also available for Bessel functions used in circular models of acoustic waveguides. We present numerical results for two different scenarios. First, a polymeric cylindrical waveguide is simulated, and second, we show simulation results of a one-sided fluid-loaded plate. For both scenarios, we compare results obtained with the proposed interval-Newton algorithm and commercial software.

Process monitoring using ultrasonic sensor systems.

Continuous in-line measurement of substance concentration in liquid mixtures is valuable in improving industrial processes in terms of material properties, energy efficiency and process safety. Ultrasonic sensor systems meet the practical requirements of a chemical sensor quite well. Currently ultrasonic sensor systems are widely used as acoustic chemical sensors to measure concentration of selected substances or to monitor the course of polymerisation, crystallisation or fermentation processes. Useable acoustic properties for the characterisation of liquid mixtures are sound velocity, sound absorption and acoustic impedance. This contribution will give a short overview of the state of the art and several trends for the use of ultrasonic sensor systems in process applications. Novel investigations show the very promising possibility to analyse liquid multi-phase mixtures like suspensions, emulsions and dispersions.