β’’-Al₂O₃ is used commercially as a Na⁺ conducting ceramic electrolyte for its high ionic conductivity (0.2-0.3 S cm^-1 at 300°C) and low materials’ costs. However, for batteries using β'’-Al₂O_3, about half of the cell resistance arises from the electrolyte itself because traditionally processed electrolyte materials are 1-2 mm thick. One can anticipate dramatic drops in cell resistance at <100 µm. However, traditional high sintering temperatures of 1600 °C/0-4 h cause rapid and excessive Na₂O loss driving formation of the β- rather than β'’-Al₂O₃ phase lowering ionic conductivities limiting final properties. Known methods of β'’-Al₂O₃ sintering commonly involve covering the sample with the same powder to reduce Na₂O loss. The quality (phase, particle size, particle morphology, etc.) of the starting powder has been shown to have a strong effect on β’’-Al₂O₃ sintering behavior.

In line with our latest success of using flame made nanopowders (NPs) to minimize the external energy input for sintering Li₇La₃Zr₂O₁₂, Li⁺ conducting ceramic electrolyte known for its difficulty in sintering, we show that the same can be achieved for β’’-Al₂O₃. In this study, β’’-Al₂O₃, TiO₂ and ZrO₂ NPs were produced by liquid-feed flame spray pyrolysis (LF-FSP). The NPs were then processed to green films (80 µm) by tape casting such that β'’-Al₂O₃ with 0, 1, 2, and 3 wt.% of TiO₂ addition is produced. TiO₂ was selected to aid sintering as Al³⁺ vacancies generated by Ti⁴⁺ doping promote the sintering process. Furthermore, low melting point (1050-1150 °C) Na₂O-TiO­₂ line compounds form inducing liquid phase sintering.

As expected, superior densification of β'’-Al₂O₃ films occurs with increasing TiO₂ wt. %. Near full densities are reached at ≥ 1360 °C/2 h with 2 and 3 wt. % added TiO₂. However β’’-Al₂O₃ content reaches only ~65 wt.% and the orientation of the c-axis is perpendicular to the film surface per XRD. The preferred conduction plane is perpendicular to the c-axis reducing net conductivity. Also, the large grain sizes seen in the 2 and 3 wt. % TiO₂ samples suggest liquid phase sintering. To perturb grain reorientation during liquid phase sintering, and also pin grain boundaries to reduce grain growth, we introduced 10 wt. % of ZrO₂ with 2 and 3 wt.% TiO₂. The added ZrO₂ may also reduce Na₂O loss rate by reducing surface exposure of β’’-Al₂O₃.

Sintering of the aforementioned samples both reduces final grain sizes and degrees of c-axis orientation. Furthermore, nearly full densities could be accessed at 1320 °C/2 h with 85 wt.% β'’-Al₂O₃. Ionic conductivities obtained equal those reported by others. Our approach is scalable and economical for mass producing β'’-Al₂O₃ films. Furthermore, the combination of nanopowders with selected additives allows sintering β'’-Al₂O₃ at 1320 °C, the lowest ever reported.

Work supported on NSF subcontract from Na₄B to University of Michigan and by a give from Mercedes-Daimler.