Special issue  

Indium oxide inverse opal is a promising new material for optical gas sensors. The photonic properties caused by the inverse opal structure can be utilized to read out the sensors’ electronical state by optical methods. The maintenance of good thermal stability of transducer material during operation is a minimum requirement. We present results on the synthesis and investigation of the structural stability of the In₂O₃ inverse opal structure up to a temperature of 550 °C (limit of substrate).

This paper investigates the effect of high-temperature and low-temperature (< 150 °C) processing conditions on the surface composition of the substrate. Furthermore, the resultant transducers from high- and low-temperature fabrication processes are compared to determine if a low-temperature processing method is feasible. For these studies a sol-gel spray-on process is employed to deposit piezoelectric ceramics onto a stainless-steel 316L substrate.

In this work, a ceramic housing was designed using the versatile low-temperature cofired ceramics (LTCC) to test this technology at temperatures above 400°C. Inside the housing, a prototype sensor chip was mounted to monitor the signal quality during an applied heat treatment up to 600 °C. All devices failed at around 450–500 °C due to material migration from the glass seal of the housing to the sensing structures of the mounted chip. Elemental analysis identified bismuth as a contaminant.

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.