Synthesis and Oxygen Mobility of Bismuth Cerates and Titanates with Pyrochlore Structure

Abstract

Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi_1.6Y_0.4Ti₂O₇ were synthesized by the citrate route; Bi₂Ce₂O₇ and Bi_1.6Y_0.4Ce₂O₇-by the Pechini method. The structural characteristics of materials after conventional sintering at 500-1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi_1.6Y_0.4Ti₂O₇, occurs after high-temperature calcination. Complex oxides Bi₂Ce₂O₇ and Bi_1.6Y_0.4Ce₂O₇ have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO₂-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes.

Keywords: bismuth cerate; bismuth titanate; oxygen mobility; oxygen separation membranes; pyrochlores.

Figure 1

XRD patterns of Bi₂Ce₂O₇ (a) and Bi_1.6Y_0.4Ce₂O₇ (b) obtained by conventional sintering at various temperatures.

Figure 2

IR spectra of Bi₂Ce₂O₇ (a) and Bi_1.6Y_0.4Ce₂O₇ (b) obtained by conventional sintering at various temperatures.

Figure 3

XRD patterns of Bi_1.6Y_0.4Ti₂O₇ sintered at various temperatures using conventional sintering (a) and radiation-thermal sintering at 1100 °C (b).

Figure 4

IR (a) and RAMAN (b) spectra of Bi_1.6Y_0.4Ti₂O₇ samples sintered at various temperatures using conventional sintering.

Figure 5

SEM micrographs of Bi₂Ce₂O₇ (a) and Bi_1.6Y_0.4Ce₂O₇ (b) obtained by conventional sintering at 1100 °C.

Figure 6

SEM micrographs of Bi_1.6Y_0.4Ti₂O₇ obtained by conventional sintering at 1300 °C for 10 h (a,b), at 1100 °C for 10 h (c,d), and radiation-thermal sintering at 1100 °C for 30 min (e,f).

Figure 7

Temperature-programmed isotope exchange of oxygen with C¹⁸O₂ in a flow reactor for bismuth cerate and titanate samples sintered at 700 °C. Points—experiment, lines—modeling.

Figure 8

Arrhenius plots for oxygen tracer diffusion coefficients acquired by TPIE data modeling for Bi₂Ce₂O₇ (1), Bi_1.6Y_0.4Ce₂O₇ (2), and Bi_1.6Y_0.4Ti₂O₇ (3) samples sintered at 700 °C compared to other oxide materials: Bi_1.6Sc_0.2Ti₂O_7−δ (4) [26], Bi_1.6Mg_0.2Ti₂O_7−δ (5) [26], Bi_1.6Zn_0.2Ti₂O_7−δ (6) [20], Zr_0.84Y_0.16O_1.92 (7) [49], La_0.8Sr_0.2MnO_3−δ (8) [50], La_0.5Sr_0.5Fe_0.7Co_0.3O_3−δ (9) [51].