In this contribution, an inexpensive and robust impedimetric NO_x sensor is presented. The impedance of a functional thick film depends selectively on the NO_x concentration in the exhaust but shows a dependency on the oxygen concentration. Therefore, an additional temperature-independent resistive oxygen sensor structure was integrated on the same sensor platform. It serves not only to determine the oxygen concentration in the exhaust, but also to correct the oxygen dependency of the NO_x sensor.

Symmetrical Pt | YSZ | Pt–NO gas sensors were produced with frit-containing and fritless Pt electrodes and fired between 950 and 1300 °C. The sensors were operated by pulsed polarization. With fritless pastes, the sensors responded significantly higher. The firing temperature affects the sensitivity only slightly. The low NO sensitivity of the frit-containing electrodes was attributed to a blocking effect at the triple-phase boundaries that inhibits the oxygen transport through the sensor.

A planar resonant radio-frequency gas sensor was equipped with an integrated heater. By simulative geometry optimization it now can be operated up to 700 °C. Sensitive materials with gas-dependent dielectric properties at higher temperatures can now be used. By coating the sensor with zeolite, ammonia could be detected. Depending on the working temperature, the sensor returns either a dosimeter signal (low temperatures) or a gas-concentration-dependent radio-frequency signal (high temperatures).

We prepared BaFe_(1-x)-0.01Al_0.01Ta_xO_3-δ (BFATx) thick films with x between 0.1 and 0.4 at room temperature using the aerosol deposition method and we measured Seebeck coefficients and conductivities between 600 and 800 °C at different oxygen concentrations. Deposited on a transducer that includes a heater, equipotential layers, and electrode structures, a dual thermoelectric–resistive oxygen sensor with almost temperature-independent characteristics of both measurands was realized using BFAT30.

Monitoring hydrocarbon concentrations in automotive exhausts is affected by flow rate changes. The signal of thermoelectric gas sensors is a thermovoltage. Its origin is a temperature difference that depends on the flow rate. To avoid this noise effect, the sensor can be installed in a defined bypass position. As shown by simulation and experiments, the gas flow around the sensor is almost turbulence-free and the signal only depends on the hydrocarbon concentration and not on the flow rate.

A planar thermoelectric gas sensor is modeled. By coupling all influences (fluid flow, gas diffusion, heat transfer, chemical reactions, and electrical properties) a model was set up that mirrors the sensor behavior precisely, as the comparison with experimental data shows. The coupling of 3-D and 1-D geometry enables to calculate the temperature distribution, fluid flow, and the gas concentration distribution in the 3-D model, while the chemical reactions are very accurately calculated in 1-D.

An FEM model is used to improve the sensor design of a Tian–Calvet calorimeter. By modifying the basic part of the sensor (a sensor disc based on low temperature co-fired ceramics), the sensitivity was increased by a factor of 3. The model was validated and the sensors were calibrated. Indium and tin samples were measured. The melting temperatures show a deviation of 0.2 K while the enthalpy was measured with a precision better than 1 %. The values for tin deviate by less than 2 % from literature.

Initial steps to apply a new ceramic multi-layer sensor for a Tian–Calvet calorimeter are shown. The FEM-developed sensor consists of stacked ceramic discs and insulation rings. The functionality of the sensor disc was proven up to 600 °C and the entire stack was tested at room temperature. The resolution was 5 µW and the sensitivity was 8.5 µV mW⁻¹. The new sensor shows similar specifications as commercial devices and presents a good starting point for future high temperature applications.

A manufacturing process for a planar binary lambda sensor is shown. By joining the heating and the sensing components via glass soldering with a joining temperature of 850 °C, a laboratory platform has been established that allows the manufacturing of two independent parts in HTCC technology with electrodes that are post-processed at lower temperatures, as is required for mixed-potential sensors. The concept has been proved by comparing the device with a commercial sensor.

Thimble-type lambda probes that are known for their robustness in harsh exhausts can also be used as an NOx sensor by applying the pulsed polarization technique. This study evaluates in detail the influence of temperature on the NO sensitivity, so that an optimum operating point can be derived. Stepwise NO concentration changes between 0 and 12.5 ppm in synthetic exhausts demonstrate the high potential of this concept.

A concept to measure in situ sulfidation of silica pellet catalysts loaded with nickel is evaluated. During sulfidation between 100 and 400°C nickel sulfides form. The electrical impedance of the pellets was recorded in situ. At first, the particles are highly insulating but during sulfidation their conductivity increases by decades. Since nickel sulfides are less conductive than nickel, the strong conductivity increase may be due to conducting percolation paths that form during sulfidation.