(95m) Atomically Deposited Sintering Aids: Assessing the Effects of Alumina Particle ALD on the Sintering and Performance of SOFC Electrolytes and Dental Ceramics
The crystal structure of zirconium oxide (ZrO2) can be tailored by the incorporation of aliovalent dopants, most commonly, yttrium oxide (Y2O3). The concentration of dopant dictates the thermodynamically stable crystal phase where 3 mol% Y2O3 (Y-TZP) stabilizes the mechanically strong tetragonal phase and 8 mol% Y2O3 (YSZ) stabilizes the oxygen-conducting cubic phase. To achieve suitable mechanical properties and ionic conductivity, Y-TZP and YSZ ceramics must be near theoretical density, requiring high sintering temperatures. Previous researchers have successfully lowered the required Y-TZP and YSZ sintering temperatures through incorporation of aluminum oxide (Al2O3) particles. While this technique has proven to reduce the sintering temperature in both systems, the presence of particulate Al2O3 inclusions in densified parts reduces the homogeneity of the material which may have deleterious effects on densification and microstructure. In this work, we report on the use of atomic layer deposition (ALD) to conformally coat individual Y-TZP and YSZ particles with the desired amount of Al2O3.Constant-rate-of-heating (CRH) experiments were conducted using a horizontal push-rod dilatometer to extract the activation energy of sintering and also gain mechanistic insights into the active diffusion mechanisms as a function of ALD film thickness. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the as-deposited thin Al2O3 films and the effects of the Al2O3 films on grain growth and microstructure during densification. Ionic conductivity and flexural strength tests were conducted to determine the effect of Al2O3 thin films on relevant material properties. Al2O3 incorporation by ALD is shown to have unique phenomenological effects on the densification of Y-TZP and YSZ as compared with conventional Al2O3 incorporation using particle-based approaches.