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Title:Sol-gel processing, perovskite phase development and properties of relaxor-based thin layer ferroelectrics
Author(s):Francis, Lorraine Falter
Doctoral Committee Chair(s):Payne, David A.
Department / Program:Materials Science and Engineering
Discipline:Materials Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Materials Science
Abstract:This thesis reports on the preparation and properties of thin layer ceramics in the Pb ((Zn$\sb{1/3}$Nb$\sb{2/3})\sb{\rm 1-x}$Ti$\sb{\rm x}$) O$\sb3$ (PZNT) and Pb ((Mg$\sb{1/3}$Nb$\sb{2/3})\sb{\rm 1-x}$Ti$\sb{\rm x}$) O$\sb3$ (PMNT) systems for capacitor and ferroelectric applications. Sol-gel methods were used to prepare thin layers by spin-casting partially hydrolysed alkoxide-based solutions on platinized Si substrates. As-deposited layers were amorphous, and formed a nonferroelectric pyrochlore phase or a ferroelectric perovskite phase, depending on the processing route. Since the perovskite structure is required for ferroelectric properties, the phase development and processing methods for perovskite formation were investigated in detail for thin layers and powders.
Crystalline phase development in sol-gel derived powders and thin layers was studied by thermal analysis and X-ray diffraction. All compositions crystallized first into a cubic pyrochlore phase. Pb$\sb2$ ((M$\sb{1/3}$Nb$\sb{2/3})\sb{\rm 1-x}$Ti$\sb{\rm x}$) $\sb2$O$\sb6$ and at higher temperatures, formed the perovskite phase Pb ((M$\sb{1/3}$Nb$\sb{2/3})\sb{\rm 1-x}$Ti$\sb{\rm x}$) O$\sb3$ (M is Mg or Zn). For powders, the gelation conditions affected the rate of perovskite formation. The unit cell parameters and formation temperatures for the pyrochlore and perovskite phases were found to depend on composition. The evolution of structure with heat-treatment was explained in terms of a general model of thermally activated processes.
With optimized processing, thin layers in the entire PMNT system could be prepared in the perovskite structure. The most effective processing method was spin-casting, followed by rapid heat-treatment to a temperature sufficiently high for the perovskite structure to crystallize, without the formation of the pyrochlore phase. Solution additives (HNO$\sb3$, NH$\sb4$OH and benzoic acid) also affected perovskite content and microstructure. Dielectric and ferroelectric properties of thin layers were determined and related to composition, microstructure and phase content. PMN rich thin layers had high dielectric constants (K) and low remanent polarization (P$\sb{\rm r}$) values (e.g., 90 PMNT: K $\sim$ 2200, tan $\partial$ $<$ 0.03; P$\sb{\rm r}$ = 3 $\mu$C/cm$\sp2$, E$\sb{\rm c}$ = 18 kV/cm), whereas PT rich thin layers had relatively high P$\sb{\rm r}$ values with lower dielectric constants (e.g., 30PMNT: K $\sim$ 500, tan $\partial$ $<$ 0.03, P$\sb{\rm r}$ $\sim$ 12 $\mu$C/cm$\sp2$, E$\sb{\rm c}$ = 40 kV/cm). Thin layers in the PMNT system have potential for thin layer capacitor and ferroelectric memory applications which can now be integrated directly onto semiconductors by sol-gel methods.
Issue Date:1991
Rights Information:Copyright 1991 Francis, Lorraine Falter
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9124413
OCLC Identifier:(UMI)AAI9124413

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