Abstract
The film thickness plays an important role in the performance of materials applicable to different technologies including chemical sensors, catalysis and/or energy materials. The relationship between the surface and volume of the functional layers is key to high performance evaluations. Here we demonstrate the thermophoretic deposition of different thicknesses of the functional layers designed using flame combustion of tin 2-ethylhexanoate dissolved in xylene, and measurement of thickness by scanning electron microscopy and focused ion beam. The parameters such as spray fluid concentration (differing Sn2+ content), substrate-nozzle distance and time of the spray were considered to investigate the layer growth. The results showed ≈ 23, 124 and 161 μm thickness of the SnO2 layer after flame spray of 0.1, 0.5 M and 1.0 M tin 2-EHA-Xylene solutions for 1200 s. While Sn2+ concentration was 0.5 M for all the flame sprays, the substrates placed at 250, 220 and 200 mm from the flame nozzle had layer thicknesses of 113, 116 and 132 µm, respectively. Spray time dependent thickness growth showed a linear increase from 8.5 to 152.1 µm when the substrates were flame sprayed for 30 s to 1200 s using 0.5 M tin 2-EHA-Xylene solutions. Changing the dispersion oxygen flow (3–7 L/min) had almost no effect on layer thickness. Layers fabricated were compared to a model found in literature, which seems to describe the thickness well in the domain of varied parameters. It turned out that primary particle size deposited on the substrate can be tuned without altering the layer thickness and with little effect on porosity. Applications depending on porosity, such as catalysis or gas sensing, can benefit from tuning the layer thickness and primary particle size.
Highlights
Wet chemical routes, vapor deposition technique, and direct deposition of the nanoparticle aerosol stream are major technologies for thin and thick film coating [1,2,3,4,5,6,7].Every technique is specific to the nature of the layer structure, thickness and mechanical stability [8]
The BET surface area is related to the average equivalent primary particle size as [30]: dBET = 6/(ρp ·SA ), where dBET is the average diameter of a spherical particle, SA represents the measured surface area of the powder in m2 /g, and ρp is the theoretical density in kg/m3
While mass of nanoparticles deposited on the substrate follows a linear trend similar to the layer thickness vs. spray time, the porosity is varying within 98–99.5%
Summary
Wet chemical routes (drop coating, dip and/or spin-coating, and screen printing), vapor deposition technique (chemical vapor deposition, physical vapor deposition, plasma deposition), and direct deposition of the nanoparticle aerosol stream (e.g., flame spray pyrolysis, spray pyrolysis) are major technologies for thin and thick film coating [1,2,3,4,5,6,7]. In the flame aerosol technology, thermophoretic deposition is the dominant layer formation mechanism and the deposition rate is directly controlled by the temperature gradient of the aerosol stream and the substrate [11,14]. Such thermophoretic deposition offers several advantages in comparison to wet chemistry processes or vapor deposition techniques including: (1) possibility of a single step gas phase deposition avoiding any post-treatment such as evaporation or drying of liquid components used; (2) self-forming aggregates during the gas phase deposition leading to crack-free layers; (3) overall short processing times (especially compared to CVD). In addition to previous work, we examine porosity of our samples
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