Thermal processing and the evolution of composition, structure and properties for sol-gel derived lead zirconate titanate thin layers
Lakeman, Charles David Edward
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https://hdl.handle.net/2142/20471
Description
Title
Thermal processing and the evolution of composition, structure and properties for sol-gel derived lead zirconate titanate thin layers
Author(s)
Lakeman, Charles David Edward
Issue Date
1994
Doctoral Committee Chair(s)
Payne, David A.
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
Research is reported for the relationships that exist between processing, composition and structure development, and properties, for sol-gel derived $\rm Pb(Zr\sb{0.53}Ti\sb{0.47})O\sb3$ (PZT) thin layers. Emphasis was placed on the effect of thermal processing conditions on the gel-to-ceramic conversion. In addition, the effects of interfacial reactions, and residual stresses on the measured properties were also considered. The purpose of the work was to contribute to the fundamental understanding of processes which occur during the high temperature treatment of sol-gel derived thin layers.
The gel-to-ceramic conversion was monitored by transmission electron microscopy, diffuse reflectance infrared spectroscopy, and X-ray diffraction. As-deposited coatings were found to be amorphous with compositional heterogeneity at the nanoscale. This is the first report which identifies the nature of the compositional heterogeneity. Structural rearrangement occurred during thermal processing and, for slow heating (50$\sp\circ$C/min), the coatings approached compositional uniformity on crystallization into the perovskite phase. However, for faster heating rates, the degree of structural rearrangement was limited, thereby decreasing the shrinkage normal to the coating, and crystallization was shifted to a higher temperature. Also, for very rapid thermal processing (5000$\sp\circ$C/min), compositional heterogeneity was retained in the coatings after crystallization.
An apparent dependence on thickness for the properties for ferroelectric PZT thin layers was observed. It was confirmed that a cover coat of a PbO-precursor deposited onto the gel-derived layers prior to firing compensated for the volatilization of PbO which occurred during heating, resulting in coatings which displayed improved properties over those prepared without a cover coat. A simple series capacitor model was investigated to interpret the data. SIMS and TEM analyses indicated that diffusion of some of the species resulted in the formation of a sub-electrode nano-crystalline phase, however, no second phases were observed between the electrode and the coating.
Measurements of the residual stress state in PZT coatings deposited onto platinized silicon substrates, and fired at 700$\sp\circ$C, indicated a tensile stress of approximately +200MPa in the plans of the coating. Thermal expansion mismatch stresses were calculated to be of the order of $-$300MPa in compression, indicating that large tensile stresses were generated during constrained shrinkage.
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