Novel tube-type LTCC transducers with buried heaters and inner interdigitated electrodes as a platform for gas sensing at various high temperatures

Novel tube-type sensor transducers manufactured in LTCC technology with inner interdigital electrodes and equipped with buried heater structures were developed for high temperature gas sensing up to 700 °C. A homogenous temperature distribution of the sensitive layer was achieved by optimization of the structures of the buried heaters by FEM modeling and validated by infrared camera measurements. The thermal time constants of the tubular transducer upon heating and cooling were evaluated. The electrical insulation resistance of the uncoated transducers is in the range of those of 96% pure alumina substrates which are commonly applied for gas sensors. The suitability of the presented tubular sensor transducers was demonstrated exemplarily by depositing a NOx sensitive layer on the screen-printed inner interdigital electrodes. By accumulating NOx molecules as nitrates this sensitive device works like a dosimeter with additionally temporal information about the actual NOx level. The novel rotational symmetry of the transducer geometry with the gas flowing inside of the tube is particularly advantageous for NOx sampling, especially at varying flow rates. The self-heated structure allows a sensitivity variation by adjusting the temperature and a periodic thermal regeneration of the storage sites. From 350 to 450 °C, the sensor signal correlates with the total amount of NOx while the NOx concentration is obtained from the signal derivative at a constant flow rate. Thereby, the sensitivity increases with temperature. However at 650 °C, the sensitive device works like a common gas sensor allowing a direct concentration detection.