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Structures of calcium-rare-earth (Ca-RE) fluorocarbonate minerals from southwest China have been investigated using selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). They are described as combinations of layers of bastnäsite-(Ce), CeFCO3 (B layers), and synchysite-(Ce), CeFCO3·CaCO3 (S layers) (Donnay and Donnay, 1953). We report here the discovery of six new, regular, mixed-layer structures in parisite-(Ce) observed using SAED and HRTEM. The symmetry, cell parameters, chemical formulae and stacking of the structural unit layers, etc., were determined for each. The regular, mixed-layer structures are formed by stacking unit layers of bastnäsite-(Ce) and synchysite-(Ce) in varying proportions along the c axis. HRTEM shows that there are different distribution modes for the Ce-F ion layers, the CO3 ion groups between the Ce-F ion layers and the CO3 ion groups between the Ce-F and Ca ion layers. The minerals may be regarded as polymorphs with the same chemical composition and the same spacing of the unit layers, but different arrangements of ion layers in the structural unit layers and different stacking sequences of the structural unit layers.
The ultrastructures of parisite-(Ce) from a rare-earth mineral deposit of Sichuan Province, southwest China, have been observed and investigated by selected area electron diffraction (SAED) and high resolution electron microscopy (HREM). Six new polytypes 4H, 10H, 16H, 18R, 24R and 30R were determined in parisite-(Ce), as were their crystal structure types, subcell (a′c‴, a′c″, a′c′) and supercell (ac) parameters, reflection conditions, polytype features and probable space groups. The HREM studies revealed the complex polytypism in parisite-(Ce) whereby the six new polytypes, formed by long-period ordered stacking, always coexist with the common polytype parisite-(Ce)-6R by syntaxy and thus constitute the complex microstructural ‘syntactic polycrystals’, the host of which is the polytype parisite-(Ce)-6R.
Kirkendall void formation at the solder/metallization interface is an important reliability concern for Cu conductors and under-bump metallization in microelectronic packaging industry, whose mechanism is still hard to be understood for different individual cases. In the present work, two typical solder/Cu-diffusing couples, eutectic SnIn/Cu and SnBi/Cu, were studied by scanning/transmission electron microscopy to investigate the microstructural evolution and voiding process after soldering and then solid-state aging. It was concluded that Kirkendall voids formed between two sublayers within Cu2(In,Sn) phase in eutectic SnIn/Cu solder joint, whereas they appeared at the Cu3Sn/Cu interface or within Cu3Sn for eutectic SnBi/Cu solder joint. Besides the effect of impurity elements, the morphological difference within one intermetallic compound layer could change the diffusing rates of reactive species, hence resulting in void formation in the reaction zone.
The microstructure of a low-carbon steel after high current density electropulsing treatment was characterized by high-resolution transmission electron microscopy. It was found that nanostructured γ-Fe could be formed in the coarse-grained steel after the electropulsing treatment. The mechanism of the formation of a nanostructure was discussed. It was thought that change of the thermodynamic barrier during phase transformation under electropulsing was a factor that cannot be neglected. It was reasonable to anticipate that a new method might be developed to produce nanostructured materials directly from the conventional coarse-grained crystalline materials by applying high current density electropulsing.
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