Synthesis and Characterization of Doped Mayenite as a Transparent Conducting Oxide

Base and doped mayenite (Ca₁₂Al₁₄O₃₃) were synthesized using both solid-state and citrate gel techniques; and fired in air and reducing atmospheres. The prepared compositions were studied using neutron and x-ray diffraction at room temperature, high temperature x-ray diffraction (HTXRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), digital optical microscopy, and Brunauer, Emmett and Teller (BET) surface area analysis. Electrically conductivity was measured on the Fe (iron) doped samples using a 2-point, after firing in a reducing atmosphere.

The HTXRD data on pure mayenite synthesized using citrate gel method and fired in air showed that the sample remained amorphous until around 750 °C (degrees Celsius) before crystallization started. The first observed phase was CaCO₃ (calcium carbonate), as the temperature increased, other phases including Ca₃Al₂O₆ (tricalcium dialuminate), and CaAl₂O₄ (calcium dialuminum) were observed but disappeared with higher temperatures while the mayenite phase increased and was eventually the only phase at 900 °C. The crystallization temperature of CaCO₃ was confirmed by thermal analysis result conducted in compressed air. However, when high temperature x-ray diffraction data were collected in reducing environment, the presence of CaCO₃ was not observed. Scanning electron microscopy results showed that sample prepared using citrate gel technique has more numerous but smaller pores compared to the solid-state prepared sample.

Using the citrate gel method more incorporation of the dopant into the mayenite structure was achieved than using solid-state synthesis. Equilibrium was reached at lower temperatures and shorter time during firing with the citrate acid method compared to the samples prepared using solid-state synthesis. Neutron diffraction data collected on mayenite with 0.3 atomic% Fe confirmed that Fe was replacing Al (aluminum) in the Ca₁₂Al₁₄O₃₃ framework and an increase in the lattice constant was observed. X-ray diffraction data also confirmed the increase in lattice constant with increasing Fe concentration. Conductivity measurements with a 2-point probe on samples containing Fe and fired in reducing atmosphere showed increased resistivities compared to pure mayenite fired in reducing atmosphere. The different Fe concentrations showed about the same amount of resistance.