Factors affecting the phase composition of 612 aluminates for Ba–W cathode

The precursor powder of 612 aluminates were synthesized by using liquid-phase co-precipitation method. The microstructure and formation mechanisms of the precursor powder were investigated through X-ray diffraction, thermal gravimetric, differential scanning calorimetry, and scanning electron microscope. The effects of the stacking states of the precursor powder, calcination temperature, and atmosphere on the phase compositions of 612 aluminates were also systematically studied. Results showed that the prepared precursor powder was a mixture of BaCO₃, BaCa(CO₃)₂, and amorphous AlOOH, which with particle size ranging from 20 to 30 nm. The stacking states of the precursor powder considerably influenced the phase composition of aluminates. The pressed precursor tablet can ensure the final phase composition of aluminates was Ba₃CaAl₂O₇. When the precursor powder was calcined in CO₂ atmosphere at 1400 °C for 2 h, the phase composition included Ba₅CaAl₄O₁₂, BaAl₂O₄, and BaCO₃. When the calcination temperature was increased, the main crystalline phase of aluminates changed from Ba₅CaAl₄O₁₂ to Ba₃CaAl₂O₇ in flowing N₂, Ar, and static air. The barium–tungsten cathode prepared by aluminates of Ba₃CaAl₂O₇ phase showed better emissivity than that of Ba₅CaAl₄O₁₂ phase. The current density of pulse emission at 1050 °C can reach 35.31 A/cm².