FIGURE 7 (a) TEM image and (b)-(e) Al, N, O, and C elemental mappings of the dark grey AlN powder sample, (f) and (g) HRTEM image and SAED pattern of the circle in (a)
Figure 7a-e are a TEM view and elemental mappings of a spherical AlN ultrafine particle of the dark grey AlN powder sample. A dark plaque in Figure 7a-e may be concave on the particle surface. As shown in Figure 7b-d, the Al and N contents are high throughout the particle, but the O element is generally concentrated in the central region of the particle of a shape like yolk. The HRTEM image and the selected area electron diffraction (SAED) spectrum of the edge of the particle are shown in Figure 7f and g. The planar spacing of 2.692 ± 0.059 Å in Figure 7f is very close to that of the AlN(100) plane (2.700 Å) [26]. In conjunction with the EDS results, the particle was determined to be AlN. During AlN powder synthesis in the CRN process, AlN nucleates on the Al2O3 particle surface. It gradually grows into a continuous AlN layer and encloses the residual Al2O3 core to form a core-shell structure [29]. The yolk-like region in Figure 7d is composed of not only residual Al2O3 but also the Al2OC mesophase between the Al2O3 core and the AlN shell. To provide more conclusive evidence, the TEM of the dark grey AlN powder is shown next.