How to Make Ceramic Gas Sensor Films at Room Temperature : The Powder Aerosol Deposition Method

Titelangaben

Moos, Ralf ; Bektas, Murat ; Hagen, Gunter ; Kita, Jaroslaw ; Schönauer-Kamin, Daniela ; Hanft, Dominik ; Exner, Jörg:
How to Make Ceramic Gas Sensor Films at Room Temperature : The Powder Aerosol Deposition Method.
2021
Veranstaltung: The 18th International Meeting on Chemical Sensors, IMCS2021 , May 30 - June 6, 2021 , online conference.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung , Vortrag )
DOI: https://doi.org/10.1149/MA2021-01561521mtgabs

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Abstract

Typically, gas sensors are ceramic devices. They are manufactured in ceramic techniques like tape technology and/or conventional thick-film techniques (typically screen-printing with subsequent firing). During firing, interdiffusion processes occur between substrate and gas sensitive film. At least partly, they may deteriorate the gas sensing properties of the functional oxides. Some materials can even hardly be processed without decomposition. Therefore, room temperature deposition techniques are advantageous.

The Powder Aerosol Deposition Method (PAD) bases on the Room Temperature Impact Consolidation (RTIC) as the film densification mechanism. This allows producing dense ceramic films without any high-temperature processes directly from an initial ceramic powder on almost any substrate material. This contribution gives examples for PAD-applications in the field of gas sensing. Although the process is rather simple, the results are very promising. Driven by a pressure difference to a vacuum deposition chamber (evacuated only to rough vacuum), the aerosol is accelerated by a slit nozzle to several hundred m/s. This aerosol jet ejects particles into the deposition chamber. Here, the particles impact on a movable substrate and get fractured into nanometer-sized fragments that are compacted by subsequently impacting particles. Fig. 1 depicts the basic principle. Further process details can be found in reviews.

Some examples for PAD-based sensors are surveyed here. Besides conventional conductometric gas sensors based on SnO and other metal oxides to detect limited components, applications for temperature independent oxygen sensors are reported, e.g. of SrTi1-xFexO3 or Alumina-doped BaFe1-xTaxO3(BFATx). Simultaneous powder aerosol co-deposition of inert and functional oxides to fine-tune the sensing
properties may be promising.

Figure 2 is a measurement protocol at 900 °C of the logarithm of the film conductivity of BFAT30 fabricated by PAD. The material is rapidly and reproducibly responding to stepwise changes of the oxygen partial pressure, pO2. In Figure 3, the log-log plot of the conductivity (log s vs. log pO2) of the sensor between 600 °C and 900 °C in the pO2 range from 0.01 to 1 bar is shown. Neither the sensitivity to oxygen (slope in the log-log-plot) nor the base line resistance changes with temperature. Log s depends linearly on log pO between 700 °C and 900 °C, showing a slope of 0.24. An improved formulation contains 1 % alumina (BFATx). Amongst the BFATx samples examined, particularly good properties were found with regard to temperature independency for BFAT25 (BaFe0.74Al0.01Ta0.25O3). Combined resistive and thermoelectric oxygen sensors with almost temperature-independent characteristics of both conductivity and Seebeck coefficient were manufactured using BFAT30. Classical tin-oxide conductometric metal oxide gas sensors were deposited by PAD at room temperature on interdigital electrodes. As expected, they show a pronounced sensitivity to nitrogen oxides at around 300 °C and to hydrogen at around 450 °C.

The powder aerosol deposition method is also suitable to produce thermistors with a negative temperature coefficient of resistance (NTCR). They may serve as sensitive detectors for pellistors.

It is possible to manufacture ceramic gas sensor films completely without any high-temperature process and directly from an initial ceramic powder on almost any substrate material. Even temperature sensors can be realized. Future research directions may focus on sensors on polymers or textiles by applying the powder aerosol deposition method.