Fabrication of nanostructured functional coatings using solution precursor plasma spraying
We are developing a solution-precursor plasma spraying (SPPS) process to form coatings with nanostructured features for applications in photocatalysis and electrocatalysis. The coatings are produced by introducing solutions of metallic salts into a high temperature plasma plume which then send a stream of molten particles into a desired substrate.
Plasma spraying is a thermal spray coating process in which usually an argon-based plasma jet is used as a source of heat to melt the coating material and project it towards the working piece. The versatility of spraying powders, solutions, and suspensions allow the production of a large variety of materials (e.g. metals, metal oxides, and non-oxide systems). In particular, solution precursor plasma spraying (SPPS) is a liquid-feedstock thermal decomposition process mostly used to produce functional oxide coatings such as TiO2, yttria-stabilized zirconia, and Ni-Fe oxides. SPPS utilizes aqueous/organic solutions of metallic salts formulated to achieve a desired stoichiometry and fed into a high temperature plasma plume. As in any other liquid-based thermal deposition method, the material synthesis involves several steps including vaporization, fragmentation, decomposition, pyrolysis, sintering, melting, splat formation and splat solidification, therefore a high temperature source is preferred to fulfil the energy requirements. Plasma spraying involves high temperatures (> 20,000 K) short reaction times (milliseconds), large particle speed (up to 2000 m/s), and high cooling rates (107 K/s). These conditions offer a unique environment to produce diverse nanostructured materials.
The available equipment is a MET-PCC(PLAS) atmospheric plasma spray system from Metallisation. It is equipped with a 50kW torch with internal and external feeding nozzles, argon is used as primary gas and nitrogen/helium as secondary gas. The liquid feeding system is comprised of two high-pressure syringe pumps (500D syringe pump from Teledyne ISCO) capable of feeding a wide range of chemical with stable flow rates from 0.001 mL min-1 up to 200 mL min-1 at pressures up to 259 bar. These are used to inject the suspension/slurry into the core of the plasma plume. The equipment is also capable of spraying powders (Atmospheric Plasma Spraying, APS), and nanoparticle suspensions (Suspension Plasma Spraying, SPS). The plasma torch is controlled by an ABB robotic arm IRB 2600. The spraying room is located at the Physics Department, and the instrument was co-financed by the Kempe Foundation and Umeå University. The equipment is being used now to produce nanostructured coatings based on transition metal oxides for photocatalysis and electrocatalysis.