Tilman Sauerwald

A High Temperature Capacitive Humidity Sensor Based on Mesoporous Silica

Capacitive sensors are the most commonly used devices for the detection of humidity because they are inexpensive and the detection mechanism is very specific for humidity. However, especially for industrial processes, there is a lack of dielectrics that are stable at high temperature (>200 °C) and under harsh conditions. We present a capacitive sensor based on mesoporous silica as the dielectric in a simple sensor design based on pressed silica pellets. Investigation of the structural stability of the porous silica under simulated operating conditions as well as the influence of the pellet production will be shown. Impedance measurements demonstrate the utility of the sensor at both low (90 °C) and high (up to 210 °C) operating temperatures.

Gas mixing apparatus for automated gas sensor characterization

We developed a computer-controlled gas mixing system that provides automated test procedures for the characterization of gas sensors. The focus is the generation of trace gases (e.g. VOCs like benzene or naphthalene) using permeation furnaces and pre-dilution of test gases. With these methods, the sensor reaction can be analyzed at very low gas concentrations in the ppb range (parts per billion) and even lower. The pre-dilution setup enables to cover a high concentration range (1:62?500) within one test procedure. Up to six test gases, humidity, oxygen content, total flow and their variation over time can be controlled via a LabVIEW-based user-interface.

Review of Portable and Low-Cost Sensors for the Ambient Air Monitoring of Benzene and Other Volatile Organic Compounds

This article presents a literature review of sensors for the monitoring of benzene in ambient air and other volatile organic compounds. Combined with information provided by stakeholders, manufacturers and literature, the review considers commercially available sensors, including PID-based sensors, semiconductor (resistive gas sensors) and portable on-line measuring devices as for example sensor arrays. The bibliographic collection includes the following topics: sensor description, field of application at fixed sites, indoor and ambient air monitoring, range of concentration levels and limit of detection in air, model descriptions of the phenomena involved in the sensor detection process, gaseous interference selectivity of sensors in complex VOC matrix, validation data in lab experiments and under field conditions.

Optimierung des temperaturzyklischen Betriebs von Halbleitergassensoren

Zusammenfassung Wir stellen ein Verfahren zur Optimierung des temperaturzyklischen Betriebs (TCO, temperature cycled operation) im Hinblick auf Sensorsignal, Sensitivität und Selektivität vor. Dieses basiert auf der Grundlage eines Sensormodells unter der Annahme, dass die Leitwertänderung durch die Besetzungsänderung mit ionosorbierten Sauerstoff auf dem Sensor hervorgerufen wird. Das Verfahren wurde mit einem SnO2-Sensor (AS-MLV, ams Sensor Solutions Germany GmbH, Reutlingen) auf einem Membransubstrat getestet, welches aufgrund der geringen thermischen Masse eine schnelle Temperaturänderung zulässt. Das Optimierungsverfahren kann sehr hohe Sensorsignale (GGas/GLuft − 1) erzielen, beispielsweise ein Sensorsignal von ca. 8000 bei 1 ppm ethanolhaltiger synthetischer Luft, die das isotherme Sensorsignal um den Faktor 800 übertrifft. Der Zusammenhang zwischen dem Sensorsignal und der Gaskonzentration kann für die meisten Zeitpunkte im TCO durch eine Potenzfunktion mit Exponenten von 0,5 bis 4 beschrieben werden. Es zeigt sich, dass direkt nach einem Temperaturabfall das Modell so stark vereinfacht werden kann, dass nur noch ein freier Modellparameter bleibt. Die Geschwindigkeit der Relaxation bei reduzierendem Gasangebot kann durch Ratenkonstanten beschrieben werden, die in guter Näherung proportional zur Gaskonzentration sind. Die Temperaturcharakteristik der Ratenkonstanten variiert für verschiedene Gase unabhängig von der Konzentration. Daher kann auch die Selektivität optimiert werden, wie am Beispiel von ethanol- und benzolhaltiger synthetischer Luft gezeigt wird.

Versatile infrared gas measurement system with tunable microstructured Fabry-Pérot Filter

Based on the combination of non-dispersive infrared (NDIR) transmission and photoacoustic absorption spectroscopy (PAS) measurements, the detection of multiple gases and their mixtures was investigated using an integrated, microstructured Fabry-Pérot Filter (FPF). The proposed versatile system can be utilized as IR spectrometer for a wide range of gas concentrations using IR transmission spectroscopy (IRTS) measurements for high concentrations and PAS for low concentrations. The system is based on low cost components: a thermopile detector for the IRTS signal, condenser microphones for the PAS signal and a microstructured IR source. The source is modulated electrically with a frequency in the range of 1 to 10 Hz, i.e. using a nonresonant operating mode of the gas chamber. The investigated, commercially available FPF allows the analysis of gases with gas specific absorption bands between 3.9 and 5.0 μm. Our IRTS system, therefore, can be adapted for many application scenarios in which two or more gases have to be detected over a wide concentration range. One example is the isotope specific detection of carbon dioxide (CO2) for the urea breath test.

Detecting trace-level concentrations of volatile organic compounds with metal oxide gas sensors

We are studying the reactions of metal oxide semiconductor gas sensors to hazardous volatile organic compounds (VOCs) at concentrations in the lower ppb and even sub-ppb range with and without interfering gases. For these measurements a gas mixing apparatus was realized providing concentrations from some ten ppt to some hundred ppb in a non-varying carrier gas stream. First results show high sensitivity (relative change in conductance) of a commercial thick film sensor, for example approx. 15 % for 500 ppt benzene and 137 % for 40 ppb naphthalene, also in the presence of ethanol (2 ppm) as interfering gas. The absolute change of conductance of the sensor for naphthalene is constant with and without interfering gas.

Percolation Effects in Metal Oxide Gas Sensors and Related Systems

The percolation model has successfully been applied to gas sensors in the past few years. We will describe which kind of sensor properties have been explained or predicted by percolation effects, as, e.g., digital sensor characteristics, tailor-made sensor properties, and resettable dosimeters. Clearly, it is a challenging and nontrivial task to reduce the whole complexity of an experimental system to fit into a mathematical model, as the percolation model. By giving a comprehensive introduction, we will point out which properties of the percolation theory are universal and therefore best suited for the comparison to the experimental results. Furthermore, practical hints for the fitting procedure will be discussed. The focus will be on the metal oxide gas sensors, but we keep in mind that there are further interesting examples for percolation gas sensors and sketch important related systems. We believe that a combined understanding of theoretical and experimental aspects will open new fields for the utilization of percolation effects for metal oxide gas sensor and beyond.

Percolation transition in the gas-induced conductance of nanograin metal oxide films with defects

We use Monte-Carlo Simulations to study the conductance switching generated by gas-induced electron trapping/-releasing in films of sintered metal oxide nanoparticles by using a site-bond percolation model. We explore the possibilities of gas sensors based on these mechanisms. In our study, we model films of different thicknesses where the conductance values of the grains (sites) and of the contacts (bonds) between these grains depend on the surface density Nr of adsorbed gas molecules from the ambient atmosphere. Below a critical density Nr=Nr,c, the system is insulating due to the interruption of current flow, either through the connecting bonds or through the grain interior. This leads to two competing critical gas covering thresholds Nr,c(bond) and Nr,c(site), respectively, that separate the insulating from the conducting phase. For Nr,c(site)>Nr,c(bond), the characteristic curve of monodisperse sensors shows a noticeable jump from zero to a finite conductance at Nr=Nr,c(site), while for polydisperse ...

Device for the detection of short trace gas pulses

Abstract A device for detection of short gas pulses at very low concentrations is presented. The approach is based on a special temperature modulation technique enabling a differential surface reduction (DSR) measurement of a metal oxide semiconductor (MOS) gas sensor. With this method, the sensor surface is highly covered with oxidized surface states at high temperature (e. g. 400 °C) initially. The temperature is then reduced abruptly to, e. g., 100 °C resulting in a state with strong excess of negative surface charge. Reactions of these surface charges with reducing gases are prevailing and lead to very high sensitivity. For the measurement a dedicated detector (electronics and fluidic system) is presented. The electronics allows a high-resolution conductance measurement of the sensitive layer and accurate temperature control. The fluidic system is examined in terms of peak shape and optimal sensor response via FEM simulations.

A novel low-cost pre-concentrator concept to boost sensitivity and selectivity of gas sensor systems

Several highly relevant applications for gas sensor systems cannot be adequately addressed today due to the limited sensitivity and selectivity of available low-cost sensors. We present a novel approach to simultaneously boost sensitivity and selectivity by using low-cost, selective micro pre-concentrators (μPC) to accumulate target molecules. Micromachined Si hotplates coated with Molecular Imprinted Polymer (MIP) or Metal Organic Framework (MOF) layers are used as μPC and heated to release the adsorbed target molecules. The μPC is mounted in a small housing adjacent to a micro gas sensor to achieve an effective gas transfer. By combining several μPCs with broadband sensors, this simple device can detect several gases with unrivalled sensitivity and selectivity.