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Influence of the Powder Crystallite Size on the Successful Film Formation Using Room Temperature Aerosol Deposition

Title data

Exner, Jörg ; Kita, Jaroslaw ; Moos, Ralf:
Influence of the Powder Crystallite Size on the Successful Film Formation Using Room Temperature Aerosol Deposition.
Event: 8th Tsukuba International Coating Symposium (TICS8) , 12.12.-13.12.2018 , Tsukuba, Japan.
(Conference item: Conference , Poster )

Abstract in another language

The fabrication of high-quality ceramic films has gained in importance in the last decades. This trend is going to continue in the future, leading to the requirement of more advanced and controllable production processes. Aerosol Deposition (AD) has received much attention during the last 20 years because of its ability to deposit a large variety of ceramic materials on a variety of substrates. The unique feature of this spray coating technique is the possibility to form dense, nanocrystalline ceramic films directly from ceramic powders without the need for a heat treatment during or after deposition.

The film deposition at room temperature is the outstanding advantage of Aerosol Deposition, however the underlying Room Temperature Impact Consolidation (RTIC) mechanism imposes several requirements to the process conditions and especially to the used ceramic powder. The powder particle size is usually stated the most important powder property with typical values around 1 µm. Also the milling and drying conditions of the powder are of interest for a successful deposition. However, further not yet known powder parameters exist that can heavily influence the deposition mechanism up to the point of no film growth. As a consequence, even powders that possess the stated specifications may not be able to build high quality ceramic films. Therefore, the success of Aerosol Deposition depends on the right choice of the powder. Many commercially available powders (e.g. Al2O3, CeO2 and ZrO2) tend to build low quality films with a weak substrate adhesion and a low strength. In addition, the deposition results may be dependent on the batch number, for that reason reproducibility can prove difficult. While typically trial and error attempts of different powders (e.g. varying manufacturer) may eventually lead to a suitable choice of powder, we chose a completely different approach for our investigations.
Our goal is to pretreat a previously not depositable powder, in this case a ceria nanopowder, in order to achieve a suitability for the deposition via the RTIC mechanism.

Ceria nanopowders were thermally treated at temperatures TPT between 900 °C und 1400 °C. All pretreated as well as the untreated powders were subsequently milled and dried to enable a comparable powder condition. Fig. 1 shows the change in powder morphology. While untreated ceria powders exhibit a platelet-like shape, grain growth becomes dominant at temperatures of 1100 °C leading to spherical grains and eventually to near single crystalline, spherical particles at 1400 °C.

Depending of the pretreatment temperature, two fundamentally different deposition behaviors occurred during Aerosol Deposition. All powders treated at 1000 °C and below build thick (up to 100 µm in thickness), yet inhomogeneous, low strength films that can be easily wiped from the silicon substrate. These mechanical properties are a result of the porous film structure (Fig. 2a). In contrast, deposition of all powders treated at TPT of 1100 °C and above lead to well adhered films with high strength. These about 1 µm thick films are fully dense and free of delamination (Fig. 2b). This indicates that the RTIC mechanism only occurred for the powders treated at high TPT, while powders treated at lower TPT still exist in their unfractured, initial shape.

The observed change in the deposition behavior is linked to the crystallite size using X-ray diffraction combined with a Rietveld refinement. For the powders, the crystallite size continuously increases with TPT from 13 nm in the untreated state up to 500 nm after a TPT of 1400 °C (Fig. 2c, filled circles). However, the crystallite sizes of the thereof sprayed films differ (hollow circles). Successfully deposited films show a significantly decreased size, by at least a factor of 15 compared to the corresponding powder. This outlines two important requirements for AD: 1. Not only particles, but especially crystallites must fracture. 2. A minimum powder crystallite size is necessary to enable the fracturing in first place.

The success of Aerosol Deposition is highly dependent on the used powder properties. In addition to the particle size, also the crystallite size plays an important role in the RTIC deposition mechanism. Using an appropriate thermal pretreatment, even previously unsuitable powders can be deposited to high quality dense films.

Further data

Item Type: Conference item (Poster)
Refereed: Yes
Institutions of the University: Faculties > Faculty of Engineering Science
Faculties > Faculty of Engineering Science > Chair Functional Materials > Chair Functional Materials - Univ.-Prof. Dr.-Ing. Ralf Moos
Profile Fields > Advanced Fields > Advanced Materials
Research Institutions > Research Centres > Bayreuth Center for Material Science and Engineering - BayMAT
Faculties > Faculty of Engineering Science > Chair Functional Materials
Profile Fields
Profile Fields > Advanced Fields
Research Institutions
Research Institutions > Research Centres
Result of work at the UBT: Yes
DDC Subjects: 600 Technology, medicine, applied sciences > 620 Engineering
Date Deposited: 09 Jan 2019 08:26
Last Modified: 09 Jan 2019 08:26