Lead-based relaxor ferroelectric films produced by the metallo-organic decomposition technique
Abstract
The lead-based relaxor ferroelectrics are a promising group of materials for producing high dielectric constant capacitors. One of the problems in processing the lead-based relaxor ferroelectrics by conventional powder processing, however, has been high lead loss at the firing temperatures which leads to the formation of the parasitic pyrochlore phase. This pyrochlore phase reduces the expected dielectric constant of the relaxor materials as much as ten times. The metallo-organic decomposition (MOD) technique has great potential for producing fine relaxor ferroelectric powders and thin films because of a higher degree of mixing, lower processing temperatures and shorter firing times. The MOD process was used to investigate the formation of Pb(Mg$\sb{1/3}$Nb$\sb{2/3}$)O$\sb3$ (PMN). From the study of PMN powders, pure perovskite PMN was synthesized directly by the thermal decomposition of metallo-organics. Also, $\sp\sim$100% perovskite PMN powder was produced at temperatures as low as 700$\sp\circ$C. The details for synthesizing the PMN powders and films were then extended to producing Pb(Fe$\sb{1/2}$Nb$\sb{1/2}$)O$\sb3$ (PFN) and Pb(Ni$\sb{1/3}$Nb$\sb{2/3}$)O$\sb3$ (PNN) films. For all of the processing conditions studied, cracking was observed for all of the relaxor films. The solid state transformation from the pyrochlore phase to the perovskite phase was determined to be the cause of the cracking. This solid state transformation could not be avoided. Calculations based on the molar volume change of the surface stress indicated the surface was in tension and on the order of 10$\sp6$ psi. Although pure perovskite relaxor films could not be made directly by the MOD process, powders produced from the decomposition of the formulation solutions, and then, screen printed and fired, can be used to produce pure perovskite films. The smaller particle size obtained in MOD powders plus the elimination of ball-milling and mixing for conventional processing makes this approach useful. Electrical properties were measured for a PMN and a PFN film. A broadening of the dielectric constant over a range of temperatures, a frequency dependence and a hysteresis was observed for both samples, confirming the samples were relaxor ferroelectrics. Similar Curie temperature ranges as observed by others were found for PMN and PFN. A dielectric constant that was an order of magnitude less than observed for bulk materials was attributed to the fine grain size (250-300 nm) of the films.
Degree
Ph.D.
Advisors
Vest, Purdue University.
Subject Area
Materials science
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