(9e) Supersonic-Impaction Printing of Flame-Made Doped-Perovskite Nanoparticles

Perovskite nanoparticles find a range of applications in emerging electronics such as transparent conductors, solar cells, light-emitting diodes and sensors such as gas sensing. Such electroactive oxides typically have melting temperatures over 1000^oC. Subsequently, their deposition, calcination and sintering also require extremely high temperatures. As a result, substrate compatibility with high temperature treatments is needed to facilitate usability and implementation of such materials. This is therefore a bottleneck for their application on substrates that are susceptible to high temperatures and oxidizing environments such as plastics or metals. More recently, the advent of additive manufacturing has shed new light on the deposition and growth of oxide materials. Among them, perovskite materials synthesized via sol-gel methods followed by inkjet and aerosol printing has shown promise for being able to be deposited on low temperature substrates. In this presentation, we will be showcasing a new concept and design for a deposition chamber intended for the supersonic impaction of aerosolized nanomaterials. Further, we will also discuss the implementation of a flame spray reactor as the source of synthesizing pure Barium Stannate (BaSnO₃) nanoparticles that are impacted using supersonic deposition to create electrically conducting perovskite thin films.

The principle of supersonic deposition is primarily based on acceleration of an aerosol-containing gas stream through a converging-diverging (de Laval) nozzle. This de Laval nozzle creates a high pressure drop across itself when downstream is pumped to moderate vacuum and upstream is maintained at a higher pressure. The pressure gradient across the nozzle accelerates the flowing aerosol to supersonic speeds and subsequently impact them onto the substrate. The kinetic energy of the particles is converted into heat upon impaction and thus the process can create a homogeneous thin film of sintered materials. Through computational fluid dynamics simulations, we found a number of parameters including the effect of gas velocities, nanoparticle density, size, trajectories and other physical properties to affect the efficiency of impaction. Unlike other precursors for competing additive manufacturing techniques such as inkjet, aerosol and electro-hydrodynamic jet printing, supersonic impaction does not require the use of any surfactant stabilized nanoparticle inks.

For this reason, flame spray pyrolysis was chosen as a scalable route for the synthesis of pure aerosolized ceramic nanoparticles without the use of any surfactants. More recently, flames are also being implemented for the synthesis of complex oxides such as tertiaries and perovskites. We found that the critical parameters that affect the flame temperature are the precursor and dispersion oxygen flow rates and enthalpy of the precursor material. A higher feed rate and precursors with high enthalpies ensure higher temperatures of the flame. Doped BaSnO₃ is an electroactive perovskite material and has a large indirect intrinsic band-gap of 3.2 eV. We will discuss our developments pertaining to the incorporation of electrical properties in the thin films of BaSnO₃via vacuum annealing to induce oxygen vacancies and incorporation of external dopants that can substitute the Ba²⁺ site (n-type with La) or Sn⁴⁺ sites (p-type with K, Cs).