Low-Temperature Sintering Processes for Ceramic-Coated Heat Exchangers

ABSTRACT: An innovative high power photonic curing process incorporating broadband light including ultraviolet (UV) is proposed for low temperature sintering of ceramic materials on metallic substrates. New high power ultraviolet (UV) flash lamps developed and integrated with a system that permits complex pulse forms in which pulse energy, duration, and frequency are controlled. By delivering a series of high power flashes having durations as short as 30 microseconds, energy densities in excess of 15 kW/cm^2 can be produced with minimal heating of the underlying substrate. This technique has been used to sinter high temperature metals printed on polymer and paper without damage to the substrates. The innovation of new UV flash lamps and process settings will allow sintering of ceramic materials such as yttria-stabilized zirconia (YSZ) that have low absorption at longer wavelengths put out by currently available flash lamps. For demonstration of technical feasibility, the UV photonic curing process will be applied to functionally graded coatings deposited via direct-write printing. Coatings consisting of a gradual transition from 316 stainless steel to ceramic YSZ will be printed on 316 SS substrates. The new enhanced-UV photonic curing process will then be applied. Coating quality will be characterized in terms of density, strain, and grain size reduction. BENEFIT: Photonic curing has been used to sinter metal inks on low temperature substrates, however, the process is not currently suitable for use with many ceramics whose optical absorption at wavelengths above 500 nm is poor. A primary benefit of this effort is that high power UV photonic curing will allow these types of ceramic materials to be rapidly sintered with minimal heating of the substrate. Technical challenges associated with conventional (equilibrium) thermal sintering approaches, such as cracking and delamination from high temperature thermal expansion, can be minimized. The process is sufficiently rapid for direct coupling with most coating and material printing processes, hence it has potential for use in high volume commercial applications such as heat exchangers, solid oxide fuel cells, and thermal barrier coatings.