The change of grain shape and grain growth behavior of $Pb(Mg_{1/3}Nb_{2/3})O_{3}-PbTiO_{3}$ (PMN-PT) by variation of composition has been investigated.
In chapter 3, the effect of variation of composition on the grain shape and grain growth of PbO-excess PMN-PT has been studied. Firstly, the effect of PT content on grain shape and growth in 40 wt% of PbO-excess PMN-PT has been investigated in the views of mixed control theory for polyhedron shape grains with rounded corners and edges dispersed in a liquid matrix. When PMN-x mol%PT (PMN-xPT; x is from 0 to 35) specimens with excess PbO are sintered at 1050℃, the equilibrium shape of grains appears to be cube with rounded corner and edge. Therefore, the flat singular surface segments coexist with the curved rough segments at this composition range. As PT content increases the area fraction of the rounded edges becomes larger which indicates that roughening proceeds with decrease of PT. When PMN, PMN-2PT, PMN-4PT, and PMN-8PT specimens are sintered, grain growth looks like normal growth, and the increase of cube of average of grain size with sintering time appeared to be approximately parabolic. However PMN-10PT specimens are sintered grains grow without any incubation of abnormal grain growth (AGG) up to 4 h, but some abnormal grains observed when sintered 12 h. At this composition range the slope appeared to change continuously between flat surface segments and curved segments. When PMN-13PT, PMN-26PT, and PMN-35PT specimens are sintered, AGG occurs from 0 h of sintering time and as PT content increase, the edges of grains become sharper even for small grains. When AGG occurs, the incubation time of AGG decreases as PT content decreases. Electron back-scattered diffraction (EBSD) analysis shows that many abnormal grains have ∑3 boundaries inside them, but some small grains have ∑3 boundaries inside them also. And some grains in PMN specimens also have Σ3 boundaries inside them. To know the role of ∑3 boundaries on the growth of PMN-PT, more detailed investigation is needed. Similar grain growth behavior is observed when PMN-xPT specimens with excess PbO are sintered at 1000℃ and 950℃. The observed grain growth behavior appears to be qualitatively consistent with the hypothesis that grain growth rate of grains of polyhedron shape with rounded edges and corners dispersed in liquid matrix can be controlled by competition between step nucleation and diffusion by variation of driving force and step free energy.
Secondly, the effect of excess MgO on grain growth of 40 wt% of PbO-excess PMN has been investigated. When excess MgO is added, the edges of grains become sharper. When amount of added MgO is 5 mol% or below, AGG doesn’t occur, but when amount of added MgO is 10 mol%, AGG occurs after 12 h. The observed grain growth behavior can be explained by mixed control theory.
In chapter 4, the effect of excess MgO on the grain shape and grain growth behavior of PMN-35PT has been investigated. When PMN-35PT specimens are sintered at 1200℃, normal grain growth occurs with equiaxial grains and slightly curved interfaces. When more than 1 mol% of excess MgO is added, abnormal grain growth occurs with the large abnormal grains developing nearly cubic shapes. The PbO-rich liquid films expected to be present at the interfaces between the abnormal grains and the small neighboring grains have flat segments, indicating that the grain-liquid interface segments are singular and hence can cause the abnormal grain growth. EBSD analysis shows that very few abnormal grains have ∑3 boundaries inside them. When leached 3 dimensional morphology abnormal grains observed by scanning electron microscopy, none of observed grains have ∑3 boundaries inside them. Similar grain growth behavior is observed when PMN-35PT and 2 mol% of MgO-excess PMN is sintered at 1100℃, but frequency of ∑3 boundaries in the 2 mol% of MgO-excess PMN specimens increases remarkably. The role of ∑3 boundaries on grain growth of PMN-35PT seems to be more important at this temperature, but to know the role of them, it is needed to know by which mechanism and at which time during grain growth ∑3 boundaries are formed.
