Sintering Phenomena of Lead Magnesium Niobate
Abstract
The sintering characteristics of lead magnesium niobate, Pb(Mg_{1/3}Nb _{2/3})O_3 (PMN), were investigated systematically in order to provide an insight into the physical and chemical changes taking place during this process. A controlled-profile sintering technique was used that provided the ability to control sintering profiles to the following extent for heating rates up to 120 ^circC/min: (1) soak temperature: +/-0.5^circC, (2) starting time at temperature (T_0): +/-10 sec, (3) heating rate: +/-2%. This permitted the tracking of densification and microstructural evolution during isothermal sintering, particularly at higher temperatures where the kinetics were rapid. High densities (97-98%) were achieved at temperatures as low as 950^circC when using a heating rate of 120^circC/min. The low sintering temperature is attributed, in part, to the high surface area fissured substructure of the PMN particles resulting from the formation of perovskite PMN via the magnocolumbite method. This led to inhomogeneous densification, and a limit to final density of 98%. It is also proposed that vapor transport influences all stages of sintering and is beneficial to the densification process by providing a source for liquid phase in the early stages, and inhibiting grain growth in all stages. Because of the slow coarsening rates, lower heating rates (investigated down to 5^circC/min.) had no discernable influence on densification kinetics or grain growth. The activation energy for densification in the intermediate stage was 58 kcal/mol and 91 kcal/mol, below and above 85% density, respectively. The change in activation energy is attributed to the preferential removal of the small pore population during inhomogeneous densification. Powders milled for extended periods of time demonstrated more rapid intermediate stage kinetics due to a reduction in the degree of inhomogeneous densification, and an optimization of the densification/grain growth trajectory. Dielectric evaluations demonstrate that dielectric constant is not influenced by grain size in fully dense samples. No influence of thermal profile on the intrinsic contribution to the dielectric constant of dense samples was observed.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT.......239C
- Keywords:
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- MAGNESIUM;
- Physics: Condensed Matter; Engineering: Materials Science