Calcium cobaltite (Ca₃Co₄O₉) is considered as one of the most promising thermoelectric p-type oxides for energy harvesting applications at temperatures above 500 °C. It is challenging to sinter this material as its stability is limited to 920 °C. To facilitate a practicable and scalable production of Ca₃Co₄O₉ for multilayer generators, a systematic study of the influence of powder calcination, Bi doping, reaction sintering, and pressure-assisted sintering (PAS) on microstructure and thermoelectric properties is presented. Batches of doped, undoped, calcined, and not calcined powders were prepared, tape-cast, and sintered with and without uniaxial pressure at 900 °C. The resulting phase compositions, microstructures, and thermoelectric properties were analyzed. It is shown that the beneficial effect of Bi doping observed on pressureless sintered samples cannot be transferred to PAS. Liquid phase formation induces distortions and abnormal grain growth. Although the Seebeck coefficient is increased to 139 μV/K by Bi doping, the power factor is low due to poor electrical conductivity. The best results were achieved by PAS of calcined powder. The dense and textured microstructure exhibits a high power factor of 326 μW/m K² at 800 °C but adversely high thermal conductivity in the relevant direction. The figure of merit is higher than 0.08 at 700 °C.

NO₂ in the ppb-range can be detected at room temperature by aluminum-doped zinc oxide, whereby the sensor devices are operated as resistive gas dosimeters. In this operation mode, the dose of an analyte is measured directly and the actual analyte concentration can be determined by mathematical differentiation. It is crucial that already adsorbed analyte species do not desorb from the sensitive material and therefore their recovery time is infinite. As classically operated metal oxide sensors are known for their low selectivity, the selectivity of the sensor is investigated. The here-presented results show that the sensor can detect NO₂ very selectively even at low concentrations in the low ppb range. Since it has already been reported that humidity can accelerate the desorption of gas molecules from metal oxides, furthermore, the influence of humidity on the dosimeter behavior is investigated. It is shown that the holding capability is negatively affected and the analyte desorbs faster in humid atmosphere.

A novel sensor operation concept for detecting ppb-level NO₂ concentrations at room temperature is introduced. Today’s research efforts are directed to make the sensors as fast as possible (low response and recovery times). Nevertheless, hourly mean values can hardly be precisely calculated, as the sensors are still too slow and show baseline drifts. Therefore, the integration error becomes too large. The suggested concept follows exactly the opposite path. The sensors should be made as slow as possible and operated as resistive gas dosimeters. The adsorption/desorption equilibrium should be completely shifted to the adsorption side during a sorption phase. The gas-sensitive material adsorbs each NO₂ molecule (dose) impinging and the sensor signal increases linearly with the NO₂ dose. The actual concentration value results from the time derivative, which makes the response very fast. When the NO₂ adsorption capacity of the sensor material is exhausted, it is regenerated with ultraviolet (UV) light and the baseline is reached again. Since the baseline is newly redefined after each regeneration step, no baseline drift occurs. Because each NO₂ molecule that reaches the sensor material contributes to the sensor signal, a high sensitivity results. The sensor behavior of ZnO known so far indicates that ZnO may be suitable to be applied as a room-temperature chemiresistive NO₂ dosimeter. Because UV enhances desorption of sorbed gas species from the ZnO surface, regeneration by UV light should be feasible. An experimental proof demonstrating that the sensor concept works at room temperature for ppb-level NO₂ concentrations and low doses is given.

The pulsed polarization technique on solid electrolytes is based on alternating potential pulses interrupted by self-discharge pauses. Since even small concentrations of nitrogen oxides (NO_x) in the ppm range significantly change the polarization and discharge behavior, pulsed polarization sensors are well suited to measure low amounts of NO_x. In contrast to all previous investigations, planar pulsed polarization sensors were built using an electrolyte thick film and platinum interdigital electrodes on alumina substrates. Two different sensor layouts were investigated, the first with buried Pt electrodes under the electrolyte and the second one with conventional overlying Pt electrodes. Electrolyte thick films were either formed by aerosol deposition or by screen-printing, therefore exhibiting a dense or porous microstructure, respectively. For screen-printed electrolytes, the influence of the electrolyte resistance on the NO_x sensing ability was investigated as well. Sensors with buried electrodes showed little to no response even at higher NO_x concentrations, in good agreement with the intended sensor mechanism. Electrolyte films with overlying electrodes, however, allowed the quantitative detection of NO_x. In particular, aerosol deposited electrolytes exhibited high sensitivities with a sensor output signal ΔU of 50 mV and 75 mV for 3 ppm of NO and NO₂, respectively. For screen-printed electrolytes, a clear trend indicated a decrease in sensitivity with increased electrolyte resistance.

Powder aerosol deposition (often abbreviated as PAD, PADM, or ADM) is a coating method used to obtain dense ceramic films at room temperature. The suitability of this method to obtain ammonia mixed-potential sensors based on an yttria-stabilized zirconia (YSZ) electrolyte that is manufactured using PAD and a V₂O₅–WO₃–TiO₂ (VWT)-covered electrode is investigated in this study. The sensor characteristics are compared with data from sensors with screen-printed YSZ solid electrolytes. The PAD sensors outperform those in terms of sensitivity with 117 mV/decade NH₃ compared to 88 mV/decade. A variation in the sensor temperature shows that the NH₃ sensitivity strongly depends on the sensor temperature and decreases with higher sensor temperature. Above 560 °C, the characteristic curve shifts from exponential to linear dependency. Variations in the water and the oxygen content in the base gas (usually 10% oxygen, 2% water vapor in nitrogen) reveal a strong dependence of the characteristic curve on the oxygen content. Water vapor concentration variations barely affect the sensor signal.