Properties of Yttrium-Doped Barium Zirconate (BZY)-Hematite Mixed Ionic-Electronic Conductor

Abstract

Yttrium-doped barium zirconate (BZY), as an electrolyte material with perovskite structure for proton conducting fuel cells, exhibits high bulk proton conductivity in intermediate temperature, high chemical stability and excellent mechanical strength. BaZr0.8Y0.2O3-δ (BZY20) precursors were prepared by co-precipitation method and calcined at 1200 ˚C to form fine BZY powders. Then after pressing the BZY20 powder into pellets, the pellets were sintered at 1400 ˚C with 1 wt% nickel oxide as sintering aid for a period of 12 hours and 20 hours. The relative density of the sintered pellets of BZY (sintering 12 hours) and BZY (sintering 20 hours) rose from 95% to 96% (with respect to density of barium zirconate which is 6.21 g/cm3). SEM micrographs of the sintered BZY20 confirms the high density. BZY-hematite composite powders with different weight ratios of BZY powders and hematite nanoparticles (1:10, 10:10, and 10:20 in weight ratio) were prepared by ball-milling in ethanol. Thus prepared powders were pressed into pellets by cold isostatic pressing and after sintering at 1300 ˚C -1400 ˚C for 12 hours, and the XRD pattern of the BZY-hematite (10:10) composite was measured and clear BZY phases and 2 undefined peaks appeared (Fig. 1). What’s more, the size of crystallites of the BZY-hematite (10:10) composite is 41.0 nm, and the size of BZY crystallites (sintering 12 hours) is 30.8 nm by Scherrer equation. SEM micrographs of BZY-hematite (10:10) composites showed clear crystal growth striations which have not been observed in BZY. The crystal growth striations on the crystallites of BZY-hematite composites may imply hematite exists in the grain-to-grain contact areas and thus affects the surface energy and growth rate of BZY. However, no hematite, maghemite, magnetite or other iron oxides phases could be identified from the XRD pattern of BZY-hematite composites. As maghemite, magnetite and other iron oxides demonstrate low thermal stability, it may suggest that hematite exists as amorphous state or some reactions occurred between BZY and hematite during ball milling and sintering process. In addition, iron-doped barium zirconate (BZF) powders were synthesized by sol-gel combustion method and the XRD pattern (Fig. 1) showed obvious perovskite structure. It is inferred that iron was doped in the B site rather than the A site in iron-doped barium zirconate. Fig. 2 shows the total conductivity of BZY (sintering 12 h), BZY-hematite (10:1) composite, BZY-hematite (10:20) composite, hematite, BZF and BZY (sintering 20 h) under humidified nitrogen (3% H2O) with platinum electrodes. Impedance spectra of BZY-hematite composite obtained under humidified nitrogen (3% H2O) with platinum electrodes from 120 ˚C -500 ˚C showed very high conductivity relative to BZY, which implies high electronic conductivity rather than proton conductivity (for BZY-hematite 1:10, conductivity is 1.77×10-4 S/cm under 260 ˚C, which is 100 times higher than BZY; for BZY-hematite 20:10, conductivity is 1.10×10-3 S/cm under 260 ˚C, which is 1000 times higher than BZY under 260 ˚C; and for BZF, conductivity is 5.01×10-5 S/cm under 260 ˚C). Besides the conductivity of BZY-hematite composites depended on the ratio of hematite. It is almost impossible to distinguish the arc of bulk resistance and the arc of grain boundary resistance at low temperatures for BZY-hematite composite, while those distinctions are obvious for BZY below 370 ˚C. With the addition of hematite, the total conductivity activation energy becomes lower. What’s more, the electrical conductivities of BZY-hematite 1:10, 10:10 and 10:20 were measured as 4.03×10-7, 3.84×10-6 and 5.48×10-2 S/m by direct current under room temperature which reinforces the BZY-hematite acts as a conductor while BZY is an insulator and hematite is an n-type semiconductor. Furthermore, it is very interesting that BZY-hematite composites (1:10, 10:10 and 10:20) exhibit ferromagnetism at room temperature which means those BZY-hematite composites can be attracted by magnets. However, neither BZY nor hematite shows ferromagnetism under room temperature. Besides, iron-doped barium zirconate exhibits similar physical and electrical properties as BZY which means it is an insulator with no ferromagnetism. Figure 1