Isabella Marr

Dual mode NOx sensor: measuring both the accumulated amount and instantaneous level at low concentrations.

The accumulating-type (or integrating-type) NOx sensor principle offers two operation modes to measure low levels of NOx: The direct signal gives the total amount dosed over a time interval and its derivative the instantaneous concentration. With a linear sensor response, no baseline drift, and both response times and recovery times in the range of the gas exchange time of the test bench (5 to 7 s), the integrating sensor is well suited to reliably detect low levels of NOx. Experimental results are presented demonstrating the sensor’s integrating properties for the total amount detection and its sensitivity to both NO and to NO2. We also show the correlation between the derivative of the sensor signal and the known gas concentration. The long-term detection of NOx in the sub-ppm range (e.g., for air quality measurements) is discussed. Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed.

Planar Zeolite Film-Based Potentiometric Gas Sensors Manufactured by a Combined Thick-Film and Electroplating Technique

Zeolites are promising materials in the field of gas sensors. In this technology-oriented paper, a planar setup for potentiometric hydrocarbon and hydrogen gas sensors using zeolites as ionic sodium conductors is presented, in which the Pt-loaded Na-ZSM-5 zeolite is applied using a thick-film technique between two interdigitated gold electrodes and one of them is selectively covered for the first time by an electroplated chromium oxide film. The influence of the sensor temperature, the type of hydrocarbons, the zeolite film thickness, and the chromium oxide film thickness is investigated. The influence of the zeolite on the sensor response is briefly discussed in the light of studies dealing with zeolites as selectivity-enhancing cover layers.

Dosimeter-Type NOx Sensing Properties of KMnO4 and Its Electrical Conductivity during Temperature Programmed Desorption

An impedimetric NOx dosimeter based on the NOx sorption material KMnO4 is proposed. In addition to its application as a low level NOx dosimeter, KMnO4 shows potential as a precious metal free lean NOx trap material (LNT) for NOx storage catalysts (NSC) enabling electrical in-situ diagnostics. With this dosimeter, low levels of NO and NO2 exposure can be detected electrically as instantaneous values at 380 °C by progressive NOx accumulation in the KMnO4 based sensitive layer. The linear NOx sensing characteristics are recovered periodically by heating to 650 °C or switching to rich atmospheres. Further insight into the NOx sorption-dependent conductivity of the KMnO4-based material is obtained by the novel eTPD method that combines electrical characterization with classical temperature programmed desorption (TPD). The NOx loading amount increases proportionally to the NOx exposure time at sorption temperature. The cumulated NOx exposure, as well as the corresponding NOx loading state, can be detected linearly by electrical means in two modes: (1) time-continuously during the sorption interval including NOx concentration information from the signal derivative or (2) during the short-term thermal NOx release.

Undoped and doped poly(tetraphenylbenzidine) as sensitive material for an impedimetric nitrogen dioxide gas dosimeter

This article presents a nitrogen dioxide (NO2) detecting gas dosimeter based on poly(tetraphenylbenzidine) poly(TPD) as nitrogen oxide (NOx) sensitive layer. Gas dosimeters are suitable devices to determine reliably low levels of analytes over a long period of time. During NOx exposure, the analyte molecules are accumulated irreversibly in the sensing layer of the dosimeter enhancing the conductivity of the hole conducting poly(TPD), which can be measured by impedance spectroscopy. Due to their possibility for low cost production by simple printing techniques and very good physical, photochemical, and electrochemical properties, poly(TPD)s are suitable for application in gas dosimeters operated at room temperature. We studied the effect of doping with a Co(III)-complex in combination with a conducting salt on the dosimeter behavior. Compared to the undoped material, a strong influence of the doping can be observed: the conductivity of the sensing material increases significantly, the noise of the signal d...

P1GS.21 - Dosimeter for Low-Level NOx Detection – Influence of the Deposition Method of the NOx Storage Film

The detection of low-level NOx concentrations and the dose of NOx for air-quality monitoring (AQM) is still a huge task and a widely discussed topic. Dosimeter-type NOx sensors detect directly the NOx dose and are advantageous considering mean value detection, linearity of sensor responses, and drift phenomena. The electrical properties of a NOx storage film, here potassium permanganate impregnated on alumina powder, depend linearly on the amount of sorbed NOx. The electrical resistance correlates very well with the amount of formed nitrate and nitrite species. The influence of the deposition method of the sensitive NOx storage material on the characteristic behavior of the dosimeter-type sensors is investigated. Aerosol-deposited dense films (ADM films) behave like porous thick-films, but typical sensor characteristics like detection limits and relative resistance changes seem to be different.

P2.8.5 Sensing of NO, NO2, and NH3 with Zeolite-Based Impedimetric Gas Sensors

In this study, the suitability of zeolites for two different sensor principles is investigated. Zeolites can change their electrical properties with the sorption of gas molecules in their microporous network structure. Therefore, they are potential materials for impedimetric gas sensors. The screened zeolites H-ZSM-5, Fe-ZSM-5, Pt-loaded H-ZSM-5, and Pt-loaded Fe-ZSM-5 are tested for their sensitivity towards NO, NO2, and NH3 at varying temperatures. The two different sensor principles are considered: the integratingor accumulating-type sensing behavior at low temperatures and a conventional gas sensor behavior at high temperatures. At high temperatures, only the expected response towards NH3 occurs, whereas at low temperatures a cross effect towards NOx could be observed for H-ZSM-5 and Fe-ZSM-5. The sensor behavior seems to be an integrating one at 200 °C for NO and NO2. By inserting elementary platinum the sensitivity towards NO and NO2 was eliminated due to a higher catalytic activity of the Pt-loaded zeolite. At temperatures of 400 °C and 500 °C, the Ptloaded zeolites show no response even not towards NH3.