Crystallography and microstructural studies of phase transformations in two ceramic systems: Dysprosia and dicalcium silicate
Kim, Youn Joong
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https://hdl.handle.net/2142/21008
Description
Title
Crystallography and microstructural studies of phase transformations in two ceramic systems: Dysprosia and dicalcium silicate
Author(s)
Kim, Youn Joong
Issue Date
1991
Doctoral Committee Chair(s)
Kriven, Waltraud M.
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
Phase transformations in two ceramic systems, Dy$\sb2$O$\sb3$ and Ca$\sb2$SiO$\sb4$, have been studied. Focuses were on understanding the transformation mechanisms of the low- to high-symmetry transformations accompanied by a large volume increase.
Modulated structures were observed in CaO-doped B-Dy$\sb2$O$\sb3$. Three modulation vectors were required to index all the reflections observed in the modulated structure: q$\sb{\bf 1}$ (001-type, $\lambda \sim$ 9.0 A), q$\sb{\bf 2}$ (200-type, $\lambda \sim$ 7.5 A) and q$\sb{\bf 3}$ ($\lambda \sim$ 40 A). Ordering of oxygen vacancies generated from the substitution of Dy$\sp{3+}$ by Ca$\sp{2+}$ and subsequent crystallographic shear were suggested to be the primary origin of the modulations. A new twin was observed in the modulated B phase. The B to C transformation was induced by mechanical grinding.
Three superlattice structures were observed: (i) the x3c-type and the x2a,2b-type in $\alpha\sp\prime\sb{\rm L}$; and (ii) the x3a-type in $\beta$. Almost all $\beta$ grains were twinned and strained. Some transformed $\gamma$ grains were retained within $\beta$ grains, producing large cracks. Domain structures were observed in $\beta$ grains: (i) two twin-related domains for samples which experienced the $\alpha\sp\prime\sb{\rm L}$ to $\beta$ transformation; and (ii) three 120$\sp\circ$ rotation-related domains for samples which experienced the $\alpha$ to $\alpha\sp\prime\sb{\rm H}$ transformation. A marked influence of the $\alpha \to \alpha\sp\prime\sb{\rm H}$ transformation on the $\beta \to \gamma$ transformation was observed. The $\beta$ to $\gamma$ transformation was induced by mechanical grinding.
Characteristics common to both systems were noted. The A $\to$ B in Dy$\sb2$O$\sb3$ and $\alpha \to \alpha\sp{\prime} \to \beta$ in Ca$\sb2$SiO$\sb4$ are probably ferroelastic. Lattices of B-Dy$\sb2$O$\sb3$ and $\beta$-Ca$\sb2$SiO$\sb4$ are inherently strained. Both structures are not stable at ambient pressure but are stable at high pressures. The strained structures have strong electrostatic charge repulsions between some cations. The higher symmetries and larger volumes of the transformed phases (i.e., B $\to$ C in Dy$\sb2$O$\sb3$ and $\beta \to \gamma$ in Ca$\sb2$SiO$\sb4$) are probably related to the process of straightening of the lattices to release strains and maximization of the volumes to release repulsions. Both transformations can be displacive but are required to overcome comparatively high energy barriers due to the breaking of some oxygen bonds in the structures.
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