Characterization of Lanthanum Zirconate Formation at the A-Site-Deficient Strontium-Doped Lanthanum Manganite Cathode/ Yttrium-Stabilized Zirconia Electrolyte Interface of Solid Oxide Fuel Cells Aijie Chen, w Gerald Bourne, Kerry Siebein, Robert DeHoff, Eric Wachsman and Kevin Jones Materials Science and Engineering Department, University of Florida, Gainesville, Florida 32603 Lanthanum zirconate (LZO) formation and growth kinetics between 1273–1673 K at the A-site (La, Sr)-deficient Sr-doped lanthanum manganite (LSM)/yttrium-stabilized zirconia (YSZ) interface were studied using high-resolution transmission elec- tron microscopy–energy-dispersive X-ray spectrometry (HRTEM–EDS). A cross-section TEM sample preparation technique was developed using a dual-beam focused ion beam and an in situ Omniprobe manipulator. The LZO pyrochlore phase was identified at the LSM/YSZ interface by TEM. The LZO is epitaxial with respect to the YSZ phase but not to the LSM phase. EDS results coupled with structural analysis sug- gest that La diffusion is a critical step in LZO formation. This study shows that LZO formation can occur even for A-site- deficient LSM with a La composition of 78 mol% (o86 mol%). The activation energy for LZO formation was found to be 16878 KJ/mol. I. Introduction T HERE are concerns regarding chemical interactions between the stoichiometric lanthanum manganite (LSM) cathode and yttrium-stabilized zirconia (YSZ) electrolyte in both fabri- cation and operation of the solid oxide fuel cell (SOFC). 1–3 Ter- nary phases, lanthanum zirconate (LZO) and/or strontium zirconate (SZO), have been observed at the LSM/YSZ interface during high-temperature treatment. 1–8 These isolation layers have a lower ionic conductivity than YSZ and therefore increase the cathode polarization. 9 LZO is the only ternary phase ob- served in cathode materials with Sro30 mol%. 1,10 It has been reported that the ternary phase formation is reduced by chang- ing the chemistry of the LSM from a stoichiometric composition to an A-site-deficient LSM. 2,10,11 LZO can be avoided if La composition is o86 mol%. This is attributed to low cation ac- tivity in the LSM. 12 Absence or retarded formation of LZO has been confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), 2 and transmission electron microscopy (TEM) analysis 13–15 at the A-site-deficient LSM/YSZ interface. Previous studies of LZO formation have been limited by either the relatively poor detection limits of XRD and SEM or by the challenge of making TEM cross-sections of the interface. A number of studies of LZO formation have focused on either very long time anneals (4100 h) at low temperature (o1473 K) or shorter anneals (o10 h) at high temperature (41473 K). A few TEM studies of LZO formation have been performed by Tricker 15 and Mitterdorfer and Gauckler. 13 Tricker has re- ported morphology evolution of the LZO phase nucleation at the LSM/YSZ interface. 15 LZO nucleates on the YSZ surface, forming bridge-like connections, 15 and LZO grows primarily into the LSM phase. Loss of contrast in the bright-field image near the LZO/LSM interface is attributed to the presence of a new Mn-deficient LSM phase at the LZO/LSM interface. Misfit dislocations have been observed at the LZO/YSZ interface. An epitaxial relationship of the LZO to the YSZ has been confirmed based on diffraction patterns taken at the YSZ/LZO interface. This epitaxial relationship depends on the crystal orientation of the YSZ. 15 Mitterdorfer and Gauckler 13 have described the LZO nucleation at the A-site-deficient La 0.85 Sr 0.15 Mn 1.02 O 3 / YSZ interface by using high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. The LZO phase has been observed although LZO is not expected to form with a La com- position o86 mol%. Mn-rich YSZ rings have been detected on top of the YSZ surface by HRTEM and AFM. They form after sintering for 2 h at 1373 K. LZO grains start to nucleate on the top of the Mn-rich YSZ rings until La 2 O 3 forms. After a sinte- ring time of 12 h at 1373 K, cube-shaped LZO islands have been observed on top of the Mn-ZrO 2 rings by HRTEM. LZO is precipitated after La 2 O 3 reacts with ZrO 2 at the triple-phase- boundary. Mitterdorfer and Gauckler have confirmed the epit- axial relationship between the LZO and the YSZ. The kinetics of the initial stages of the LZO formation has been fully studied at the A-site-rich La 0.85 Sr 0.15 Mn 0.98 O 3 /YSZ interface. However, the ability to measure the thickness of the LZO layer by AFM is compromised if the acid-etching process removes any of the LZO layers 13 or if the sample is too thick to accurately measure the initial stages of LZO formation by TEM. 15 There are no in- depth kinetics studies on the initial stages of LZO formation at the A-site-deficient LSM/YSZ interface in the literature. This study reports a new method of preparing TEM cross- sectional samples of the LSM/YSZ interface by focused ion beam (FIB). The A-site-deficient LSM has a composition of La 78 mol%. LZO formation and kinetics of the initial stages of LZO formation at the La 0.78 Sr 0.20 MnO 3d /YSZ interface are studied using HRTEM/energy-dispersive X-ray spectrometry (EDS) analysis. II. Experimental Procedure (1) Sample Preparation A symmetric SOFC unit was made by screen-printing LSM on both sides of the YSZ samples. The electrolyte used in the work contained 8 mol % yttria, the thickness of the polycrystalline YSZ was 150 mm, and a 10 mm  20 mm sample was prepared from it using a tape cast method by Marketech International Inc. (Port Townsend, WA). LSM ink, with a composition of La 0.78 Sr 0.20 MnO 3d , was provided by Nextech Materials Ltd. (Lewis Center, OH). The LSM ink was screen-printed on both sides of the YSZ with a square area of 64 mm. 2 A drying step was performed in a Fisher Isotemp drying oven (200F, Fisher Scientific Inc., Pittsburg, PA) at 393 K for 2 h. After drying, two sets of samples were sintered in a Lindberg/Blue high-temperature box furnace (Lindberg/Blue M1973K Box Furnace, Gruenberg, T. Gur—contributing editor The authors would like to thank the United States Department of Energy for funding under project number DOE project DE-AC05-76RL01830 and Nextech for supplying cath- ode inks for use in this work. w Author to whom correspondence should be addressed. e-mail: Chen_aijie@cat.com Manuscript No. 23759. Received September 18, 2007; approved May 5, 2008. J ournal J. Am. Ceram. Soc., 91 [8] 2670–2675 (2008) DOI: 10.1111/j.1551-2916.2008.02524.x r 2008 The American Ceramic Society 2670