Effect of CaF2 on the sintering and thermal expansion of La0.85Sr0.15Cr0.95O3

Abstract

Lanthanum chromite based perovskite have been widely accepted as ceramic interconnects and as protecting layers for metallic interconnects in solid oxide fuel cells (SOFCs) because of thier high electronic conductivity, chemical stability in both oxidizing and reducing atmospheres as well as their chemical and thermo-mechanical compatibility with other cell components under operating conditions [1, 2]. Strontium-doped LaCrO3 has emerged as a primary candidate for SOFC interconnects over other alkaline earth oxides such as CaO and MgO [3]. From the viewpoint of compatibility of thermal expansion among SOFC components, Sr-doped lanthanum chromite can match the coefficient of thermal expansion (CTE) to the Y2O3-stabilized ZrO2 (YSZ) electrolyte using less than 20 mol% Sr [4]. Lanthanum chromite interconnects must be a dense in order to separate fuel gas from the oxidizing agent. They are usually sintered above 1,600 C to reach a 94% relative density [5]. From cost considerations associated with commercialization, it is desirable for all components of SOFCs to be co-sintered in air to reduce manufacturing costs. However, this may cause severe reactions among the components at high temperature. Therefore, lowering the sintering temperature of the LaCrO3-based interconnect is favored. However, the volatilization of CrO3 in LaCrO3 at high temperature in air makes it difficult to sinter. Some researchers have attempted to improve the sinterability of LaCrO3 powders in different ways. Sakai et al. [6] found that chromium deficient lanthanum calcium chromites (La0.7Ca0.3Cr1)yO3, y = 0.02) could be sintered to 94% theoretical density at 1,200 C in air. Small particle sizes of the starting powders prepared by chemical process are usually beneficial to lower the sintering temperature [7]. Doping with sintering additives is another effective method to densify LaCrO3. A small amount of Sr3(VO4)2 [8] was found to produce high density LaCrO3-based perovskite materials sintered at 1,550 C in air. Alternatively, low melting point eutectics, such as LaF3, YF3 and MgF2, up to 8–10 wt% were added to densify LaCrO3 at 1,400 C in air [9]. The aim of the work described here is to experimentally investigate the sintering and thermal expansion behaviors of La0.85Sr0.15Cr0.95O3 (LSC) ceramics by using calcium fluoride as an additive. Strontium-doped lanthanum chromite powders were synthesized using traditional ceramic processing methods. The starting powders La2O3, SrCO3, Cr2O3 (>99.9 wt%) were mixed with distilled water using ZrO2 milling media in a ball mill. After drying, the powders were calcined at 1,000 C prior to adding the sintering additive. Then various amounts of CaF2 (0–0.9 wt%) were added to the calcined powders. The mixed powders were milled again and dried. After granulitization, the powders were pressed into pellets 20 mm in diameter and 2 mm in thickness under a pressure of 72 MPa. The green bodies were then sintered at selected temperatures with a heating rate of 2 C/min in air and a holding time of 3 h. The particle size distribution of the calcined powders was measured using a particle-size analyzer (NSKC-A, Nanjing University of Technology, Nanjing, China). The bulk densities of the sintered pellets were measured by Archimedes method with de-ionized water. The crystal structure of the sintered samples was analyzed using X-ray diffraction (XRD, D/max-b, Rigaku) with CuKa radiation in the X. Ding AE L. Guo (&) College of Materials Science and Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, People’s Republic of China e-mail: lc-guo@163.com J Mater Sci (2006) 41:6185–6188 DOI 10.1007/s10853-006-0161-1