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Abstract
Powder aerosol deposition (PAD) is a unique ceramic spray coating method that produces dense and well-adhering thick-films directly at room temperature, without requiring any heating or sintering. After the successful film formation, mechanical film properties like hardness or plasma resistance are remarkably good. However, when it comes to electrical properties like permittivity or electrical conductivity, the nanocrystalline structure of PAD films combined with high internal strains deteriorates partly the characteristic properties. The electrical conductivity may already be present within the as-deposited films. However, it is mostly lowered by several orders of magnitude. Therefore, a thermal post-deposition annealing is oftentimes required. In this work, electrically conducting films produced by powder aerosol deposition are surveyed based on published data. Their microstructural and electrical behavior during the post-deposition annealing treatment is summarized and reasons for the lowered electrical conductivity are identified. Additionally, the processes taking place during annealing, which eventually allow to regain bulk-like functional properties, are examined. A universal annealing behavior is found that leads to a quantitative recommendation for the suitable film annealing temperatures to regain the electrical conductivities.
Highlights
Powder aerosol deposition (PAD) is a unique ceramic spray coating in the field of energy storage and conversion for applications in stationary or mobile batmethod that produces dense and well-adhering thick-films directly at room tery systems, in capacitors, thermoelectric temperature, without requiring any heating or sintering
For all functional PAD films already published in literature, a post-deposition annealing led to a high electrical conductivity of the films, not depending on the type of the materials’ predominant charge carriers
It was shown that for all published conductive functional ceramic films produced by powder aerosol deposition, the electrical conductivity in the as-deposited state is reduced, independent of the predominating conduction mechanism
Summary
This section summarizes the state-of-the-art of powder aerosol deposition with regard to the typical used setups and process parameters as well as to typical film morphologies. Wang et al observed an about 700 nm thick, randomly distorted anchoring layer when alumina films were applied on soft aluminum substrates, while tougher stainless steel substrates showed significantly smaller sizes around 200 nm.[66] for very hard alumina or sapphire substrates, Schubert et al found no visible anchoring layer and a nearly flat filmto-substrate interface was observed.[67] New studies by Khansur et al suppose that a change in surface chemistry and polarity as well as a physical interaction of the impacting particles with the substrate material promote a good interface and anchoring layer.[68] Naoe et al found evidence of the formation of covalent bonds in the interface between a copper substrate and alumina particles.[69] It is expected, that the formation of the anchoring layer can be viewed independently from the processes occurring during RTIC including film build-up and densification through particle–particle interactions during impact This supports the idea that the deposition is divided in two subsequent stages.[11,12] The first impacting particles clean the surface and shape the anchoring layer, possibly associated with embedding a monolayer of particles. Based on the gained knowledge, we intend to achieve a more holistic and detailed understanding of the influence of a thermal post-treatment on electrically conducting PAD films
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