Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.
LaCoO 3 is a ferroelastic perovskite-type oxide. It has been shown to undergo creep at room temperature. LaCoO 3 responds to stress by changing its domain structure, resulting in formation of spontaneous strain. The microstructure of a sample of polycrystalline LaCoO 3 with history of stress was investigated using Transmission Electron Microscopy (TEM). It was compared to an unstrained sample to determine what changes are produced. TEM analysis has shown an increase in defect density as well as the appearance of atomic scale ordering. The causes of the observed ordering and their relation to ferroelastic behavior are explored. IntroductionLanthanum Cobaltite, LaCoO 3, has been studied for its potential use in solid oxide fuel cells, oxygen separation membranes, reduction catalysts, and oxygen sensors [1][2][3][4][5][6][7][8] . At higher temperatures it exhibits ionic-electronic conductivity 9 . Additionally, changes in its magnetic susceptibility around 80K and 500K suggest corresponding spin state transitions, the nature of which has been investigated in numerous studies [10][11][12][13][14][15][16][17][18] . More recently, and to a lesser extent, the mechanical properties of LaCoO 3 have been studied 19,20 . Understanding of its mechanical properties will be essential for the development of practical applications for the material. For example, when LaCoO 3 based ceramics are to be used as solid electrolytes and membranes, cracking caused by thermal instabilities could seriously compromise the membranes 20 . LaCoO 3 is a perovskite type oxide, which has general form ABO 3 . The ideal perovskite structure is cubic. However, due to differences in the ionic radius of La and Co a slight distortion results making LaCoO 3 rhombohedral. Its nearly cubic structure, however, allows it to be thought of and indexed as a pseudo-cubic. Fig. 1 shows a pseudo-cubic reprentation of a unit cell. The pseudo-cubic lattice backbone is made of Lanthanum. The Co centered in each cell as well as the O centered on the each face of the cube make up the CoO 6 octahedra.Of particular interest, it has been reported that LaCoO 3 undergoes room temperature creep 21 . Typically, ceramics undergo creep-the continuous deformation of a solid under stress with time-only at temperatures at least as high as about half their melting point 22 (the melting point of LaCoO 3 is about 1800° C). The ability of LaCoO 3 to undergo room temperature creep is indicative of its ferroelastic behavior. The application of stress in LaCoO 3 as with other ferroelastic materials results in a hysteresis loop in the stress-strain behavior. By comparison, so-called "paraelastic" materials exhibit a linear stress-strain behavior. Furthermore in ferroelastic materials application of strain can result in spontaneous strain, which remains in the material after all external stress is removed. This is the result of the reorientation of ferroelastic domains induced by stress. A recent study modeled the creep behavior of LaCoO 3 during the loading and unloading of stress, inco...
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