(516f) Flash Sintering of Coated Powders Fabricated By Particle Atomic Layer Deposition for Ceramics Manufacturing

Authors: 
O'Toole, R., University of Colorado Boulder
Yoon, B., University of Colorado Boulder
Gump, C., ALD NanoSolutions, Inc.
Ghose, S., Brookhaven National Laboratory
Raj, R., University of Colorado Boulder
Weimer, A. W., University of Colorado Boulder
Yttria-stabilized cubic zirconia (YSZ) is a common electrolyte material for solid oxide fuel cells (SOFC) due to its moderate oxygen-ion conductivity and chemical stability at high temperatures. To maximize ionic conductivity, YSZ ceramics must be near theoretical density which conventionally requires placing the sample in a furnace and slowly increasing the furnace temperature to ~1450°C over a few hours. This requires significant energy use and can adversely affect other layers of the SOFC during manufacturing. Alternatively, flash sintering has been demonstrated to densify YSZ at furnace temperatures as low as 750°C in a few seconds, resulting in cost savings, higher ionic conductivities, and smaller grain sizes as compared to conventional processing. During flash sintering, an electric field is applied across the sample and at a minimum threshold furnace temperature, the sample conductivity and power dissipation rapidly increase causing densification. Typically, small quantities of aluminum oxide (Al2O3) are added to YSZ ceramics by mechanical mixing to alter grain growth behavior and increase the final density. In this work, particle atomic layer deposition (ALD) was used to uniformly coat YSZ particles in thin films of amorphous Al2O3, homogeneously dispersing the Al2O3 prior to flash sintering. Analysis of the flash sintering behavior revealed that the Al2O3 thin film reduced the electrical conductivity of the starting powder more significantly than an equivalent Al2O3 concentration added as particles, enabling adjustment of the onset flash temperature with minimal secondary phase addition. Small quantities of Al­2O­3 increased the sample relative density at a constant sample temperature, suggesting that the Al2O3 acts as a sintering aid and enhances the densification mechanism. This is supported by in situ XRD measurements, where the dissolution of Al2O3 into the YSZ lattice is observed and suggests that Al2O3 changes the chemical environment of the YSZ lattice and grain boundaries to enhance densification. The flash sintering of core/shell powders fabricated by particle ALD results in dense parts with fine grain sizes and has a significant cost/performance advantage over conventional processing.