Abstract:Phase transformations and transient morphologies are examined as exothermic formation reactions self--propagate across Al/Ni nanolaminate films. The rapid evolution of these phases and sub--µm morphological features require nanoscale temporal and spatial resolution that is not available with traditional in situ electron microscopy. This work uses Dynamic transmission electron microscopy (DTEM) to identify intermetallic products and phase morphologies as exothermic formation reactions self--propagate in nanolaminate films grown with 3:2, 2:3, and 1:1 Al/Ni atomic ratios. Single--shot, diffraction patterns with 15 ns temporal resolution reveal that the NiAl intermetallic forms within 10 nanoseconds of the reaction front's arrival in all three types of films and is the only intermetallic phase to form as the reactions self--propagate and quench very rapidly. Concurrently, time resolved imaging reveals a transient cellular morphology in the Al--rich and Ni--rich foils, but not in the equiatomic films.The cellular features in the Al--rich and Ni--rich films are attributed to a cooling trajectory through a two--phase field of liquid + NiAl.

Metal-doped manganese oxides have been used as precursors of cathode materials for rechargeable lithium batteries due to their high capacity. A core-shell-structured material has been designed to improve the cycle life and safety of lithium batteries [1]. In this study, Co or Al-doped manganese oxides with core-shell structure were synthesized by co-precipitation method involving the precipitation of hydroxide particles from cobalt sulfide solutions. The grain structure and composition variation of metal-doped Mn3O4 particles were studied by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS). We also probed the electronic structure of the precursor materials at the nanometer scale by means of scanning transmission electron microscopy and electron energy loss spectroscopy. Figure 1 shows cross-sectional STEM images of the Co or Al-doped manganese oxides obtained by coprecipitation process. The particles showed the core-shell structure with smaller grains inside and bigger grains outside, and they coalesce into bigger particles. The grain size inside the particles is ~ 20nm and outside ~5μm. In the case of Co-doped manganese oxides, the concentration of cobalt inside the particle is constant and outside increases towards the surface. In the case of Al-doped manganese oxides, the concentration of aluminum is constant inside and outside. EELS analysis probed that inside the particle, O K-edge shows the typical shape of O K-edge of Mn 3 O 4 and outside, the first peak of O K-edge decreases. In the case of Co-doped manganese oxides, the ratio of the integrated intensities of the L 3 and L 2 white lines (L 3 /L 2 , the valency of the transition metals [2.3]) inside the particle was 2.8 of the typical value for Mn 3 O 4 and outside 3.0 ( Figure 2) It is found that the outer layer of higher cobalt concentration has lower oxidation state. In the case of Al-doped manganese oxides, the valency (L 3 /L 2 ) of the transition metals inside the particle was 2.9 and outside 2.8 (Figure 3).