Spray Pyrolysis‐Aerosol Deposition for the Production of Thick Yttria‐Stabilized Zirconia Coatings

Abstract

Aerosol deposition (AD) is a coating technique wherein particles are impacted onto a target substrate at reduced pressures, and supersonic particle impact velocities lead to coating consolidation. The limiting step in AD application is often not supersonic deposition operation, but aerosolization of powder particles with the proper size distribution; the translational impact velocity is strongly size‐dependent. It is demonstrated that by directly synthesizing particles in the gas phase, size‐controlled ceramic particles can be injected into AD systems. This in situ formation step obviates the need for particle aerosolization. Ultrasonic spray pyrolysis (USP) is applied to produce yttria‐stabilized zirconia (YSZ), and USP is directly coupled with AD to produce consolidated, thick, YSZ coatings on metal substrates. USP‐AD yields YSZ coatings on stainless steel and aluminum substrates with porosities <0.20, which grow to thicknesses beyond 100 μm. Aerodynamic particle spectrometry and electron microscopy reveal that the depositing particles are 200 nm–1.2 μm in diameter, though each particle is composed of nanocrystalline YSZ. Supporting computational fluid dynamics calculations demonstrate that the YSZ particle impact speeds are above 300 m s−1. Thermal conductivity measurements demonstrate that USP‐AD coatings have conductivities consistent with those produced from high‐temperature processes.

Sub‐Micron Thickness Self‐Limiting Electrospray Deposition via Voltage Bias of Spray Target

“…AD aerosolizes micro-/nano-sized particles at supersonic speeds toward a substrate, resulting in a dense, line of sight coatings between 1 and 100 μm. [28,29] Additionally, dip coating has reported thickness ranges between 0.01 and 10 μm and is a generally common and facile process for generating thin films. [30] However, the proposed technique using ESD has many advantages, namely, the coating process is inherently more localized as the droplets are charged and deposit onto a grounded substrate, thus increasing specificity as well as minimizing material waste.…”

Powder Aerosol Deposition and Polymers: Is There Hope for a Common Future?

“…[67] In recent work, the conventional AD system was extended with an in-line powder production unit. This upstream unit could be used for ultrasonic spray pyrolysis (USP), [75,76] as shown in Figure 8a. In USP, droplets of a precursor salt solution are fed into a high-temperature furnace reactor via a nebulizer in which the solvent evaporates and the remaining salt particles oxidize.…”

“…[76] This leads to a defined particle size distribution which can be controlled via the operating parameters of the reactor and the nebulizer. [76] For example, Ghosh et al used tin (II) and Song Influence of an electric field on reflection deposition. The morphology of a PAD film formed on substrate 2 changed from dense to porous when an electric field of þ250 V was applied between the electrodes, while at the same time, a large number of secondary particles were deposited on the grounded electrode.…”

“…Very uniformly distributed particles sizes were obtained for a furnace temperature of 1000 °C (Figure 8c). [76] Dense nanocrystalline films were produced for both SnO 2 and YSZ using this method. [75,76] This modification of the PAD process has several advantages on different levels.…”

“…[76] Dense nanocrystalline films were produced for both SnO 2 and YSZ using this method. [75,76] This modification of the PAD process has several advantages on different levels. First, the chemical composition of the precursor could be changed in situ, which in a conventional process would involve a series of powder preparation steps (grinding, drying, sieving, heat treatment); second, the particle size and degree of crystallinity could also be adjusted, making it easier to optimize process parameters and thus possibly improve the DE.…”