Study On Sintering Processes And Ionic Conducting Mechanism In Ce_0.9Gd_0.1-xBi_xO_1.95-δ Solid Electrolytes

Solid oxide fuel cell（SOFC） is a highly efficient energy conversion device, known as the green energy technology in 21 st century. Among them, the electrolyte is the most important component, which determines the overall performance of the cells. Compared with the YSZ electrolyte, Gd2O3 doped CeO₂（GDC） exhibist superior ion conductivity property at intermediate and low temperature. However, GDC is difficult to achieve its dense requirements even calcined at 1500 ℃ for 10 h. Due to the low melting point of Bi₂O₃ and its weak Bi-O bond, GDC doped by Bi₂O₃ can improve its sintering property and ionic conductivity. Therefore, the purpose of this paper is to improve the sintering properties of the samples and analyzing the ionic conducting mechanism of Bi₂O₃ doped GDC(Ce_0.9Gd_0.1-xBi_xO_1.95-δ).The nanoscale GDC（x=0） and GBDCx（x=0.01-0.05） powder was prepared by co-precipitation. The results show that the shrinkage at 1470 ℃ increase from 14.2% to 22.7%, with the relative density from 78.9% to 99.1% as the Bi3+ doping content increase from 0 to 5%, demonstrating that the densification process of GBDC materials was significantly improved. The sintering temperature decreases by 200 ℃ due to Bi₂O₃ doping. The activation for densification of GBDC0.04 is 419 k J mol-1 by Arrhennius plots, which is much lower than that of 745 k J mol-1 for GDC. The activation energy for densification of GBDC0.04 by MSC is 750 k J mol-1, 555 k J mol-1 and 322 k J mol-1 at three region, respectively. The average value is much lower than that of 725 k J mol-1 for GDC because the sintering property is improved. According to the grain growth kinetics model, the index of grain growth of GDC and GBDC0.04 is 3 and 4, respectively, which indicates that the grain growth of GBDC is inhibited.Impedance spectra shows that all the samples have their optimum sintering temperature. Over-sintering leads to the electrical properties decrease. The activation energy for grain boundaries conductivity of GDC-1500 that is higher than that of GDC-1400 by 0.05 e V. But there is no effect on the activation energy for grain conductivity. The optimal sintering temperature of GBDC decreases from 1300 ℃ to 1200 ℃, with increasing x from 0.02 to 0.05. GBDC_0.02-1300 presents an electrical conductivity of 8.1×10-4 S cm-1 at 400 ℃, four times than that of GDC-1400. The effect of sintering temperature on grain boundary blocking effect is studied according to the Mott-Schottky model. The results showed that the over-sintering leads to the increase of Schottky barrier height and the decrease of the vacancy concentration within the space charge layer. For example, the barrier height of GBDC0.02 at 1300 ℃, 1400 ℃and 1500 ℃ is 0.19 V, 0.21 V and 0.24 V, respectively. And the values of（?） is-0.70,-1.68 and-2.72, respectively. Furthermore, the average binding energy of M-O decreased due to the low binding energy of Bi-O（BEBi-O＜BEGd-O＜BECe-O）, while the elastic strain increase as a result of the large mismatch in the radius of Bi3+ and host Ce4+. These lead to the relatively high conductivity（σ700℃=0.05 S cm-1） and low total activation energy of 0.71 e V when x is 0.02. This is due to the vacancy concentration within the grain boundary of GBDC0.02-1300 is higher than that the of GDC-1400, the values of （?）is-0.7 and-1.33, respectively.The DC electric field assisted sintering technology is applied to further reduce the GBDC0.02 sintering temperature. The Tonset gradually decreases with the increases of the DC electric field intensity. It is 848, 753, 706, 659 ℃, respectively, at the E of 60, 80, 100, 120 V cm-1. The current and linear shrinkage increases sharply at flash point. The conductivity of the sample is 0.032 S cm-1 at 700 ℃, with the grain size of 71 nm at the electric field of 100 V cm-1 and the limited current density of 5 A cm-2. The shrinkage rate is 9.1 % min-1, which is much larger than that without an applied electric field（0 V cm-1） of 0.39 % min-1. According to blackbody radiation theory, the difference between the specimen and the furnance temperature ΔT is 482 ℃ under this electric field condition. This increase as the electric field and the limited current density increased. According to the thermoelectric coupling effect, the temperature distribution of a sample in 100 V cm-1 is simulated by finite element analysis, explaining the reason that the rapid densification at low sintering temperature and short sintering time during DC field is the local Joule heat.