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.