Interface roughening transition and grain growth in $BaTiO_3$ and NbC-Co

Abstract

At low temperature, interfaces are atomically flat and ordered. As temperature increases, thermal fluctuation in the form of disordered steps appears due to entropy increase and the interface structure becomes more disordered, finally atomically rough at transition temperature. This is called interface roughening transition. Two model materials are selected to investigate the effect of interface roughening transition on grain growth. The perovskite material $BaTiO_3$ is used for various electronic components because of its excellent dielectric and piezoelectric properties and $BaTiO_3$ single crystals have been fabricated by solid-state grain growth. It is a good model material to investigate polycrystalline grain growth because various grain growth modes including normal, abnormal, and twin-assisted growth occur in solid-state $BaTiO_3$. NbC-Co is one of the cemented carbides which have the outstanding wear-resistance and high-temperature mechanical properties and manufactured by liquid-phase sintering. Since NbC particles dispersed in transition metal liquid have various interface structures and grain growth behaviors depending on temperature, additives, and ball-milling, NbC-Co is good model material to investigate the correlation between surface structure and grain growth. In Chapter 2, the effect of temperature on interface structures, grain shapes, and grain growth of NbC particles dispersed in Co-rich liquid matrix has been studied. When an NbC-30wt%Co powder compact is sintered at various temperatures where NbC grains (with small amounts of Co) coexist with liquid Co-NbC matrices, the NbC grains undergo surface roughening transition with temperature increase and the grain growth changes from abnormal to normal growth. When sintered at 1400 ℃, the grains are polyhedral with sharp edges (and corners) and grow abnormally because their singular surfaces move by nucleation of surface steps. When sintered at 1600 ℃, the edges become round indicating the surfa...