Cobalt additive for lowering the sintering temperature of yttria-stabilized zirconia

At the present time the most popular solid electrolyte chosen for use in solid oxide fuel cells (SOFC) is a solid solution of ZrO2 with 8 mol % Y2O3 (8YSZ). To fabricate many cell configurations it is necessary to sinter the electrolyte to full density while maintaining porosity in a support which is required for gas access to the cells. This is difficult with presently available YSZ powders since they require temperatures in the range 1300–1700 ◦C (typically 1400 ◦C) for densification. High sintering temperatures also result in coarsegrained microstructures having poor mechanical properties [1]. Therefore, there is considerable benefit to be obtained from lowering the sintering temperature of the YSZ electrolyte since this would greatly facilitate the manufacturing process. However, it is important that the residue of any sintering aid does not degrade the electrical properties of the YSZ and impair its function in the SOFC. There are various processing routes and techniques that can be used to reduce the sintering temperature of ceramics. Such methods include sol gel, hydrothermal synthesis and microwave sintering. Each technique has its particular merits, but the common draw-back for each is the relatively high cost of industrial scale up. Potentially the cheapest and easiest technique for reducing sintering temperature is to add sintering aids. Feng and Goodenough [2] investigated Sr2Ga2O5 as a sintering aid for YSZ. The system SrO-Ga2O3 has a eutectic at 1270 ◦C for the composition Sr2Ga2O5 [3]. A YSZ sample containing 3 mol% Sr2Ga2O5 achieved 96% theoretical density at 1350 ◦C. The doped sample had lower bulk conductivity, but a higher grain boundary conductivity enhanced the total ionic conductivity by approximately 10% at 800 ◦C. Grain size in the doped sample was considerably larger than in the specimen prepared without additive. Bi2O3 is a well known sintering aid for YSZ. Keizer [4] doped YSZ with additions of Bi2O3 (1–3 mol%) and lowered the sintering temperature from 1697 to 1077 ◦C. The densification was explained by a liquid phase sintering mechanism. Kim and Kim [5] studied the effect on the microstructure and electrical conductivity of YSZ doped with 0.7–3.5 mol% Bi2O3. Analysis of sintered specimens revealed the residue of a Bi-containing liquid phase which aided densification. A dihedral angle greater than 90 ◦ indicated that the liquid had not penetrated all grain intersections and thus sintering was attributed to both liquid phase and solid state processes. It was noted that the phase stability of the cubic fluorite structure was effected by the additions of Bi2O3 and monoclinic and tetragonal minority phases were formed. However, Bi2O3 is a particularly reactive oxide and tends to form unwanted compounds with other materials in the SOFC, such as the electrodes. It has been reported that Cu and Mn both enhance the densification rate of tetragonal CeO2-ZrO2 solid solutions [6]. Shi et al. [7] found that additions of copper oxide lowered the sintering temperature and increased grain growth of YSZ due to formation of a eutectic liquid phase between CuO(Cu2O)-Y2O3-ZrO2 above 1130 ◦C. Although CuO additions upto 0.3 mol% promoted densification, additions in excess were deleterious to sintering. Seidensticker and Mayo [8] related this limit to the solubility of the additive at the grain boundary. Almost all of the CuO on addition to YSZ was found to be segregated to within 1 to 2 nm of the grain boundary. Impedance spectroscopy of CuO doped YSZ ceramics [9] have revealed that the additive decreased both the bulk and grain boundary conductivities. Recently, Kleinlogel and Gauckler [10, 11] reported the sintering of Ce0.8Gd0.2O1.9 at temperatures as low as 900 ◦C using transition metal (Ni, Cu, Co) oxide additives. Work at Imperial College [12] has confirmed these results and extended them to Ce0.9Gd0.1O1.95 (CGO). Co3O4 was found to be a particularly effective additive in that it not only assisted densification, but also enhanced ionic conductivity in the range in which it was measured (below 500 ◦C). The mechanisms of these effects are still unclear. Hartmanova et al. [13] studied the effect of additions of Co of up to 0.8 cation % on the conductivity of 12YSZ. They found an enhancement in conductivity with a maximum at 0.08 cat % Co. There was also some evidence that the Co enhanced the sintering of the YSZ. In this paper we investigate the effect of higher cobalt additions on the sintering of 8YSZ and the resulting effect of the additive on the electrical conductivity. Commercially available 8YSZ (Tosoh) was used as the starting powder to which the dopant was added in the form of nitrate (5N grade Co(NO3)2 · 6H2O, Aldrich) dissolved in ethanol. Atomic absorption spectroscopy was used to determine the quantity of cobalt in solution. The desired stoichiometric quantities of YSZ and cobalt nitrate in ethanol were mixed and lightly ground in a mortar and pestle for 30 min. The resulting paste was then oven dried at 50 ◦C and subsequently reground. Following calcination at 650 ◦C for 1 h, the powder was sieved through a 38 μm stainless steel