Microstructure and electrical properties of conductive ceramics produced by cold sintering process

Electronic and ionic conductive ceramics with excellent electrical properties, including indium tin oxide (ITO), alumina-doped zinc oxide (AZO), gallium-doped garnet-type Li_7–3xGa_xLa₃Zr₂O₁₂ (xGa-LLZO) and NASICON-type Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), are attractive for the applications in optoelectronic devices and rechargeable batteries. In this project, a new developed cold sintering process (CSP) was applied to produce the conductive ceramics with significantly reduced sintering temperatures.

AZO semiconducting ceramic with relative density of 98.3% was produced by cold sintering at optimum CSP conditions of 250 ºC, 300 MPa, 2M of acetic acid, and 60 min. The as-CSPed AZO samples presented dense microstructure with small average grain size of 0.55 μm and high phase purity. The CSPed AZO ceramics attained low resistivity of 4.20 × 10^-3 Ω·cm after post-annealed at 900 ºC for 3 h in argon attributing to the increased oxygen vacancies and the facilitated crystallisation. Comparing with conventional sintered AZO ceramics, the CSPed AZO ceramics showed much higher sinterability and lower resistivity at the same processing temperature.

ITO ceramics with relative density of 98.6% and average grain size of 2.0 μm were fabricated through cold sintering at 250 °C for 1 h under 300 MPa with subsequent post-annealing at 1400 °C for 4 h. The cold-sintered ITO samples consisted of nanocrystalline phase and amorphous phase. The subsequent post-annealing process not only facilitates crystallisation of the amorphous phase and further densification, but also promotes the development of uniform microstructure and electronic transport properties, resulting in low resistivity of 3.63 × 10^-4 Ω·cm.

0.5Ga-LLZO solid-state electrolyte with relative density of 79.2% and ionic conductivity of 8.88 × 10^-5 S cm^-1 was produced by cold sintering at 250 ºC and 300 MPa for 1 h with Li-solution (20M LiNO₃ and 7M LiOH) as transient solution. Utilising the Li-solution with high concentrated Li⁺ and pH value significantly restrained proton exchange and improved the ionic conductivity. After post-annealed at 1100 ºC for 4 h, high relative density of 92.2% and ionic conductivity of 5.08 × 10^-4 S cm^-1 were achieved, due to the reduction of grain boundary resistance.

LATP solid-state electrolyte with relative density of 86.9% and ionic conductivity of 1.83 × 10-6 S cm-1 was produced by cold sintering at 250 ºC and 300 MPa for 1 h with DI water as transient solution. Post-annealing at 900 ºC for 1 h induced further densification and grain growth, resulting in the increase in density to 88.0% and ionic conductivity to 1.32 × 10-3 S cm^-1. Microcracks were observed in post-annealed ceramics at 1000 ºC and above because of the high thermal expansion anisotropy.