Preparation, Crystallization and Properties of Amorphous Lead Iron Tungstate Ceramics
Kim, Nam Kyoung
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https://hdl.handle.net/2142/71709
Description
Title
Preparation, Crystallization and Properties of Amorphous Lead Iron Tungstate Ceramics
Author(s)
Kim, Nam Kyoung
Issue Date
1988
Doctoral Committee Chair(s)
Payne, David A.
Department of Study
Ceramics Engineering
Discipline
Ceramics Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Abstract
Research is reported for the preparation of amorphous lead iron tungstate ceramics (Pb(Fe$\sb{2/3}$W$\sb{1/3}$)O$\sb3$, PFW) by a twin-roller quenching method; and on the properties, crystallization behavior and evolution of microstructure on heat treatment.
A high quenching rate of $\sim$10$\sp6$ K/s was required to obtain the material in the amorphous state. The amorphous nature of the prepared flakes was confirmed by x-ray diffraction and thermal analyses. Kinetic data analysis was carried out to interpret the crystallization behavior. A low value of the glass transition/melting point ratio and a high value of the frequency factor for initial crystallization indicated a high degree of metastability in the as-prepared material. An Avrami exponent of 3.0 was determined, which suggested bulk nucleation and growth, and which was confirmed by electron microscopy.
The crystallization behavior and development of microstructure were investigated by x-ray diffraction, differential scanning calorimetry, and scanning electron microscopy. The perovskite structure formed after several intermediate structures and reactions. Two new metastable phases were obtained from the prepared flakes after heat trearments at 395 and 475$\sp\circ$C, respectively. The perovskite phase evolved after heat treatment at 760$\sp\circ$C, where a microstructure of uniformly sized cubic crystals developed. Densification on crystallization at an elevated temperature was realized with an almost pore-free internal microstructure. Thus the possibility of using amorphous precursors for the reduced temperature processing of dense ceramic materials was demonstrated.
Electrical property measurements were carried out on the amorphous flakes and sintered pellets. A relatively high value of the dielectric constant (K' $\approx$ 44) was obtained for the amorphous material at room temperature, which was attributed to the retention of short-range order nearest neighbor configuration of oxygen octahedra. Sintered and crystallized materials had diffuse phase transitions with relaxor behavior, whereas the amorphous material was frequency and temperature stable, as would be expected from structure-property relations.
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