Atomic Scale Analysis of Cubic Zirconia Grain Boundaries
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ABSTRACT The core structures of two symmetric tilt [001] grain boundaries in yttria- stabilized cubic zirconia are determined by Z-contrast imaging microscopy. In particular, near-X=13 (510) and Y=5 (310) boundaries are studied. Both grain boundaries are found to be composed of periodic arrays of basic grain-boundary structural units, whose atomic structures are determined from the Z-contrast images. While both grain boundaries maintain mirror symmetry across the boundary plane, the 36' boundary is found to have a more compact structural unit than the 24* boundary. Partially filled cation columns in the 24' boundary are believed to prevent cation crowding in the boundary core. The derived grain boundary structural models are the first developed for ionic crystals having the fluorite structure. INTRODUCTION Grain boundaries in yttria-stabilized cubic-zirconia (YSZ) have important ramifications for the macroscopic mechanical and electrical properties of the material [1]. While several studies have been conducted to understand the structure and chemistry of grain boundaries in YSZ [2], none have reported detailed grain boundary core structures (i.e. atomic positions). Here we provide a detailed experimental analysis of two high-symmetry YSZ grain boundaries, the near-Z=13 (510) and the 1=5 (310), corresponding to 24' and 36.8° symmetric tilt [001] grain boundaries, respectively. The grain boundaries are studied by Z-contrast scanning transmission
electron microscopy (STEM), which reveals the projected cation sublattices parallel to the tilt axis. Based on these images, structural models of the grain boundaries are derived. Although the tilt angles between the two boundaries (24' and 36.8*) differ by only 13', two completely different grain boundary cores are observed. The details of the grain boundary core structures will be discussed in detail below. EXPERIMENTAL The YSZ bicrystals were purchased from Shinkosh Co., Ltd., Japan. The near- Z=13 (510) was a 24' [001] tilt boundary, only 1.4* away from the perfect X=13 orientation of 22.6'. The second bicrystal, within the growth tolerances (--0.5°), was a perfect Y= 5 (310), or a 36.8' [001] tilt YSZ bicrystal. The tilt angles were confirmed by electron diffraction as discussed below. TEM samples were prepared from the bulk bicrystals by standard preparation techniques. First, a 2x1 mm rectangular plate containing the grain boundary was cut by a diamond wheel from the bulk specimen. The plate was mechanically polished to approximately 40 jim in thickness and then dimpled to about 15 gm. Finally, the specimens were thinned to electron transparency by ion milling with 3kV Ar ions while cooled with liquid N2. The Z-contrast STEM imaging was carried out on the 300kV VG Microscopes HB603U dedicated STEM at Oak Ridge National Laboratory equipped with a high-resolution objective 323 Mat. Res. Soc. Symp. Proc. Vol. 589 © 2001 Materials Research Society
lens capable of forming a probe 0.13 nm diameter. Transversely incoherent images were formed with a high-angle annular dark
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