On the evolution of structure and composition in sol-gel-derived lead zirconate titanate thin layers
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The evolution of structure and chemical distribution in sol-gel derived Pb(Zr0.53Ti0.47)O3 thin layers was monitored by x-ray diffraction, analytical electron microscopy, and diffuse reflectance Fourier transform infrared spectroscopy. Electron microscopy confirmed the as-deposited coatings were amorphous with short-range order. Medium-range order developed on heat treatment, and chemical heterogeneity was observed at the nanoscale. The extent of compositional heterogeneity decreased with increasing temperature. Above 500 °C, the coatings crystallized into an intermediate phase which converted to the perovskite phase above 600 °C.
I. INTRODUCTION A significant issue in polymeric sol-gel processing is the transformation of the metalloorganic gel to the metal oxide (i.e., ceramic) on heat treatment. We address this important point in this paper. The behavior of a gel on heat treatment can be considered in three stages: (i) drying and pyrolysis of organics, (ii) structural rearrangement and densiflcation, and (iii) crystallization. The particular relationship of one stage to another is dependent on the conditions of gel formation (which strongly influence the structure of the gel1) and the heat-treatment conditions (which affect the kinetics and thermodynamics of the processes which occur at high temperatures).2 In this paper, we report a systematic investigation of the evolution of structure and composition in sol-gel-derived lead zirconate titanate (PZT) thin layers. Analytical techniques used include x-ray diffraction (XRD), diffuse reflectance Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). One of the frequently cited advantages of sol-gel processing is the improved chemical homogeneity that may be achieved by mixing at a molecular level in solution. This assumes either (i) the formation of a mixed metal precursor in which all the constituent metal ions are included with the desired stoichiometry in a single molecular precursor, or (ii) an intimate mixture of monometallic species such that metal ions are distributed uniformly throughout the solution, and remain so during subsequent processing steps. In addition, the concept of a molecular building block approach to advanced
a 'Current
address: Department of Materials, University of California, Santa Barbara, Santa Barbara, California 93106.
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J. Mater. Res., Vol. 10, No. 8, Aug 1995
ceramics is founded on the first premise, in that structural units found in the final material can be observed in the precursor.3 In some systems, this has been shown to be a realistic concept. For both LiNbO 34 and Ba4Ti|3O30,5 the isolation of single crystals of mixed metal alkoxides or oxo-alkoxides with the desired cation ratio has been reported. However, in the Pb(Zr,Ti)O3 (PZT) system, there is a growing body of evidence that the structure of molecular precursors does not include the required "building blocks", and that the resulting gel structure is not homogeneous at the molecular level. This has been demonstrated for both et
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