Tham khảo tài liệu 'ferroelectrics material aspects part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Growth and Characterization of Single Crystals of Potassium Sodium Niobate by Solid state Crystal Growth 95 During growth of the single crystal in the conventional furnace single crystal growth matrix grain growth and matrix densification take place simultaneously. During crystal growth pores in the matrix can be picked up by the moving single crystal matrix interface. If the pores then separate from the interface they will become trapped in the single crystal. The size of the trapped pores increases with crystal growth distance. This is probably due to pore coalescence in the matrix before the crystal matrix interface reaches them. Application of an external pressure during crystal growth has two benefits. Firstly during the first stage 975 C 50 MPa for 2 h the poly crystalline matrix is densified. Application of an external pressure promotes densification without promoting grain growth Chiang et al. 1997 . The sintering temperature can therefore be reduced allowing the matrix to be densified without much grain growth or single crystal growth. Growing the single crystal in an already dense matrix increases the density of the crystal Fisher et al. 2007a . Secondly during the second stage 1100 C 50 MPa for 100 h the KiCuNbsOzj liquid phase sintering aid melts and penetrates the grain boundaries leaving behind pores which must be eliminated. The applied pressure increases the driving force for shrinkage of these pores within the matrix and also of pores that become trapped within the crystal Kang and Yoon 1989 . Effect of sintering aid on crystal growth and composition The effect of the amount of sintering aid on single crystal growth matrix grain growth and single crystal composition was investigated Fisher et al. 2008b . Single crystals were grown from NbC 3 powders with additions of 0 and 2 mol KiCuNbsOzj using 001 -oriented KTaOs seed crystals. Before the crystal growth experiments samples were pre-densified by hot-pressing at 975 c 50 MPa for 2
