Morphology and Domain Structure of Unsupported PbTiO 3 Thin Films Prepared by Sol-Gel Process
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ABSTRACT The morphology and domain structure of unsupported PbTiO 3 thin films with fine grains ( preferred orientation perpendicular to the film surface, since the intensity of (110) ring is much higher than that of others.
Fig. 1 Electron diffraction pattern of a 2-coat film fired at 6501C. Fig. 2 gives the bright field TEM micrographs of the thin films. Fig. 2a, 2b, and 2c show 4coat, 2-coat, and 1-coat films fired at 6501C, respectively, whereas Fig. 2d shows 2-coat film fired at 550 C. As shown in Fig.2, the grains are not fully and closely clamped, especially for the thinner films. Careful examination shows that the junctures of grains are in most cases a very thin amorphous layer and a few micropores. Embedded pores within the grains as described by Modak et al. [13], Hsueh et al. [14], and Goral et al. [15] were not found in our films. All grains are nearly spherical and their sizes are homogenous throughout the same film. Grains become rapidly smaller as the thickness of the films or the annealing temperature decrease. Domains in bulk ceramic materials have been studied extensively [1-5]. They usually occur as lamellar microtwins of the tetragonal phase oriented in < 110> direction. Since the tetragonal
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Fig. 2 TEM micrographs of the (a) 4-coat, (b) 2-coat, and (c) 1-coat films fired at 6501C, and (d) 2-coat films fired at 5501C.
distortion of PbTiO3 is high, domains are easily formed and imaged, even in the fine-grained thin films. The sample in Fig. 2a has a grain size ranging 120-180 nm. Most of the grains show single domain, while domain walls are clearly seen in a number of grains and their structure is very similar to that observed in bulk ceramics. The domain walls usually terminate at the grain boundaries and they are caused by twinning along (110) planes. The domain width is measured to be 5-20 nm, much less than that of bulk ceramics (larger than a few microns). When the grain size decreases to 90-120 nm (see Fig. 2b), the typical lamellar domain is rarely seen. Instead, in some grains domain walls are often curved and irregular (as indicated by arrows), while more singledomained grains were observed as compared to Fig. 2a. As grain size decreases further, for example, to 50-70 nm in Fig. 2c, the number of such single-domained grains increases and the domain patterns are more irregular in the multi-domain grains. The enlarged micrograph of the grain indicated by an arrow is shown at the lower right comer of Fig. 2c. When grain size further decreases (40-50 nm), as shown in Fig. 2d, almost all grains are single-domained, whereas the irregular domain patterns appear only in a few grains. An enlarged grain with curved domains is shown at the lower right comer of Fig. 2d. It was also observed that the smaller the grains, the higher the percentage of single-domained grains, even in the same film.
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The bending domain boundaries were also observed in BaTiO3 ceramics and single crystals [16,17], but the curvature was much slighter than that in the fine grains in Fig. 2b, 2c, and 2d. It i
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