The Powder Aerosol Deposition Method - Making Ceramic Gas Sensor Films at Room Temperature
Authors: Moos Ralf , Bektas Murat , Hagen Gunter , Kita Jaroslaw , Schönauer-Kamin Daniela , Hanft Dominik , Exner Jörg 
Published: May 2020

Introduction

Most 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) [1]. During firing, between substrate and gas sensitive film interdiffusion processes occur. They may (at least partly) deteriorate the gas sensing properties of the functional oxides. Some materials can even hardly be processed without decomposition. Therefore, room temperature deposition techniques may be advantageous.

The Powder Aerosol Deposition Method

The Powder Aerosol Deposition Method (PAD) bases on a film densification mechanism called Room Temperature Impact Consolidation (RTIC). 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.
The results are very promising, despite the process is rather simple. Driven by a pressure difference to a vacuum deposition chamber (evacuated only to a 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 the reviews [2], [3], or [4].

Results and Discussion

Some examples for PAD-based sensors are surveyed here. Besides conventional conductometric gas sensors based on SnO2 and other metal oxides to detect limited gaseous components, applications for temperature independent oxygen sensors are reported, e.g. of SrTi1-x
Fe
x
O3 [5], [6] or BaFe1-x
Ta
x
O3 (BFTx) [7]. Simultaneous aerosol co-deposition of inert and functional oxides to fine-tune the sensing properties may be promising [6].
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 pO2 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 [8].
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 [9].
With PAD, solid electrolyte electrochemical gas sensors have also been successfully manufactured [10]. A nitrogen oxide sensor using the novel pulsed-polarization method, which attracted much attention recently with YSZ as solid electrolyte, can now be operated at lower temperatures due to the use of a Bismuth-based fast oxide ion conductors (Fig. 4).
The powder aerosol deposition method is also suitable to produce thermistors with a negative temperature coefficient of resistance (NTCR) [11]. They may serve as sensitive detectors for pellistors.

Conclusions

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.

References

[1] N. Barsan, et al., Metal oxide-based gas sensor research: How to?, Sensors and Actuators B: Chemical, 121, 18-35 (2001); doi: 10.1016/j.snb.2006.09.047
[2] J. Akedo, Aerosol Deposition of Ceramic Thick Films at Room Temperature: Densification Mechanism of Ceramic Layers, Journal of the American Ceramic Society, 89, 1834-1839 (2006); doi: 10.1111/j.1551-2916.2006.01030.x
[3] D. Hanft, et al., An Overview of the Aerosol Deposition Method: Process Fundamentals and New Trends in Materials Applications, Journal of Ceramic Science and Technology, 6, 147-182 (2015); doi: 10.4416/JCST2015-00018
[4] M. Schubert, et al., Powder aerosol deposition method — novel applications in the field of sensing and energy technology, Functional Materials Letters, 12, 1930005 (2019); doi: 10.1142/S1793604719300056
[5] K. Sahner, et al., Assessment of the novel aerosol deposition method for room temperature preparation of metal oxide gas sensor films, Sensors and Actuators B: Chemical, 139, 394-399 (2009); doi: 10.1016/j.snb.2009.03.011
[6] J. Exner, et al., Tuning of the electrical conductivity of Sr(Ti,Fe)O3 oxygen sensing films by aerosol co-deposition with Al2O3, Sensors and Actuators B: Chemical, 230, 427-433 (2016); doi: 10.1016/j.snb.2016.02.033
[7] M. Bektas, et al., Aerosol-deposited BaFe0.7Ta0.3O3-
δ for nitrogen monoxide and temperature-independent oxygen sensing, Journal of Sensors and Sensor Systems, 3, 223-229 (2014); doi: 10.5194/jsss-3-223-2014
[8] M. Bektas, et al., Combined resistive and thermoelectric oxygen sensor with almost temperature-independent characteristics, Journal of Sensors and Sensor Systems, 7, 289-297 (2018); doi: 10.5194/jsss-7-289-2018
[9] D. Hanft, et al., Powder pre-treatment for aerosol deposition of tin dioxide coatings for gas sensors, Materials, 11, 1342 (2018); doi: 10.3390/ma11081342
[10] J. Exner, et al., Pulsed Polarization-Based NOx Sensors of YSZ Films Produced by the Aerosol Deposition Method and by Screen-Printing, Sensors, 17, 1715 (2017); doi: 10.3390/s17081715
[11] M. Schubert, et al., Characterization of Nickel Manganite NTC thermistor films prepared by Aerosol Deposition at room temperature, Journal of the European Ceramic Society, 38, 613-619 (2018); doi: 10.1016/j.jeurceramsoc.2017.09.005

Figure 1